Make await a keyword again.

Reasoning:
In JavaScript `await` is a keyword *only* inside async functions.
```javascript
let await = 1 // OK ✅

async function wait() {
  let await = 1 // NOT OK ❌
}
```

The plot twist is that the newer browsers (chrome or node) support top-level await. (By implicitly wrapping everything in an async function?)

```javascript
let await = 1  // NOT OK, with top level await ❌
```

This makes me think:
* We can replicate JavaScript parser; inside async function expressions, use of `await` as identifier will result into a syntax error
  - Downside: this feels like a responsibility for another part of the compiler, not the parser? This is already implemented in cristianoc/rescript-compiler-experiments#1, so we would also be doing double work.
  - Other downside: if we ever allow top-level await, then we just implemented the above for nothing.
* Allow `await` as a "non-keyword" everywhere with some "tricks".

```javascript
let await = 1
let x = await // Ok, now it gets tricky. Does this start an "await"-expression?
      = await fetch(url) // could be this
      = await() // could be this
      = await await() // or even this
      = await // or we might just be assigning the identifier `await` to `x`

```

 Seems like we can infer the following rules for an `await expression`:
   - space after `await`
   - next token is on the same line as `await`
   - next token indicates the start of a unary expression

 But this "breaks down" in the case of
 ```javascript
 let x = await - 1
 // could be a binary expression: "identifier" MINUS "1"
 // could also be an await expression with a unary expression: `await` `(-1)`
 ```
These whitespace sensitive rules kinda feel super arbitrary. Which makes me think that introducing `await` as a keyword is an ok compromise.

Author: Maxim

src/res_core.ml

@@ -2023,7 +2023,7 @@ and parseOperandExpr ~context p =
       when isEs6ArrowExpression ~inTernary:(context = TernaryTrueBranchExpr) p
       ->
       parseAsyncArrowExpression p
-    | Lident "await" -> parseAwaitExpression p
+    | Await -> parseAwaitExpression p
     | Lazy ->
       Parser.next p;
       let expr = parseUnaryExpr p in
@@ -2751,13 +2751,6 @@ and parseBracedOrRecordExpr p =
     let loc = mkLoc startPos p.prevEndPos in
     let braces = makeBracesAttr loc in
     {expr with pexp_attributes = braces :: expr.pexp_attributes}
-  | Lident "await" ->
-    let expr = parseAwaitExpression p in
-    let expr = parseExprBlock ~first:expr p in
-    Parser.expect Rbrace p;
-    let loc = mkLoc startPos p.prevEndPos in
-    let braces = makeBracesAttr loc in
-    {expr with pexp_attributes = braces :: expr.pexp_attributes}
   | Uident _ | Lident _ -> (
     let startToken = p.token in
     let valueOrConstructor = parseValueOrConstructor p in
@@ -3130,19 +3123,15 @@ and parseAsyncArrowExpression p =
   | _ -> assert false
 
 and parseAwaitExpression p =
-  let startPos = p.Parser.startPos in
-  match p.token with
-  | Lident "await" ->
-    let awaitLoc = mkLoc startPos p.endPos in
-    let awaitAttr = (Location.mkloc "await" awaitLoc, Parsetree.PStr []) in
-    Parser.next p;
-    let expr = parseUnaryExpr p in
-    {
-      expr with
-      pexp_attributes = awaitAttr :: expr.pexp_attributes;
-      pexp_loc = {expr.pexp_loc with loc_start = awaitLoc.loc_start};
-    }
-  | _ -> assert false
+  let awaitLoc = mkLoc p.Parser.startPos p.endPos in
+  let awaitAttr = (Location.mkloc "await" awaitLoc, Parsetree.PStr []) in
+  Parser.expect Await p;
+  let expr = parseUnaryExpr p in
+  {
+    expr with
+    pexp_attributes = awaitAttr :: expr.pexp_attributes;
+    pexp_loc = {expr.pexp_loc with loc_start = awaitLoc.loc_start};
+  }
 
 and parseTryExpression p =
   let startPos = p.Parser.startPos in

src/res_grammar.ml

@@ -151,7 +151,7 @@ let isExprStart = function
   | Underscore (* _ => doThings() *)
   | Uident _ | Lident _ | Hash | Lparen | List | Module | Lbracket | Lbrace
   | LessThan | Minus | MinusDot | Plus | PlusDot | Bang | Percent | At | If
-  | Switch | While | For | Assert | Lazy | Try ->
+  | Switch | While | For | Assert | Await | Lazy | Try ->
     true
   | _ -> false
 
@@ -257,9 +257,9 @@ let isJsxChildStart = isAtomicExprStart
 let isBlockExprStart = function
   | Token.At | Hash | Percent | Minus | MinusDot | Plus | PlusDot | Bang | True
   | False | Float _ | Int _ | String _ | Codepoint _ | Lident _ | Uident _
-  | Lparen | List | Lbracket | Lbrace | Forwardslash | Assert | Lazy | If | For
-  | While | Switch | Open | Module | Exception | Let | LessThan | Backtick | Try
-  | Underscore ->
+  | Lparen | List | Lbracket | Lbrace | Forwardslash | Assert | Await | Lazy
+  | If | For | While | Switch | Open | Module | Exception | Let | LessThan
+  | Backtick | Try | Underscore ->
     true
   | _ -> false
 

src/res_token.ml

@@ -1,6 +1,7 @@
 module Comment = Res_comment
 
 type t =
+  | Await
   | Open
   | True
   | False
@@ -111,6 +112,7 @@ let precedence = function
   | _ -> 0
 
 let toString = function
+  | Await -> "await"
   | Open -> "open"
   | True -> "true"
   | False -> "false"
@@ -210,6 +212,7 @@ let keywordTable = function
   | "and" -> And
   | "as" -> As
   | "assert" -> Assert
+  | "await" -> Await
   | "constraint" -> Constraint
   | "else" -> Else
   | "exception" -> Exception
@@ -238,9 +241,9 @@ let keywordTable = function
   [@@raises Not_found]
 
 let isKeyword = function
-  | And | As | Assert | Constraint | Else | Exception | External | False | For
-  | If | In | Include | Land | Lazy | Let | List | Lor | Module | Mutable | Of
-  | Open | Private | Rec | Switch | True | Try | Typ | When | While ->
+  | Await | And | As | Assert | Constraint | Else | Exception | External | False
+  | For | If | In | Include | Land | Lazy | Let | List | Lor | Module | Mutable
+  | Of | Open | Private | Rec | Switch | True | Try | Typ | When | While ->
     true
   | _ -> false