Make backend key data optional (vapor#296)

Rename `PostgresCastingError` to `PostgresDecodingError` and make pub…

…lic (vapor#286)

Fixes vapor#278.

Co-authored-by: Fabian Fett <fabianfett@apple.com>

Add `hasColumn` to `PostgresRandomAccessRow` (vapor#270)

Fix missing links (vapor#255)

Refactor PSQLRowStream to make async/await easier (vapor#201)

### Motivation

`PSQLRowStream`'s current implementation is interesting. It should be better tested and easier to follow for async/await support later.

### Changes

- Make `PSQLRowStream`'s implementation more sensible
- Add unit tests for `PSQLRowStream`

### Result

Adding async/await support becomes easier.

Faster decoding, thanks to fewer bound checks. (vapor#203)

Add proper support for `Decimal` (vapor#194)

* Use `PostgresNumeric` for `Decimal` instead of String
* Make `Decimal` conform to `PSQLCodable`
* Fix support for text decimals
* Add integration test for decimal string serialization
* Test inserting decimal to text column

Co-authored-by: Gwynne Raskind <gwynne@darkrainfall.org>

ByteBuffer extension, prevent naming conflicts (vapor#204)

### Motivation

Because of https://bugs.swift.org/browse/SR-15517, we might run into naming conflicts with SwiftNIO, once apple/swift-nio#1990 lands.

### Changes

- Prefix all ByteBuffer utility methods

### Result

Chances of breaking code reduced.

DataRow without allocation; DataRow as Collection; RowDescription top…

… level (vapor#198)

This is a cherry pick of vapor#188.

### Modifications

- `DataRow` and `RowDescription` have been moved out of the `PSQLBackendMessage` namespace. This allows us to mark them as `@inlinable` or `@usableFromInline` at a later point, without marking everything in `PSQLBackendMessage` as `@inlinable`
- `DataRow` does not use an internal array for its columns anymore. Instead all read operations are directly done on its ByteBuffer slice.
- `DataRow` implements the `Collection` protocol now.

### Result

One allocation fewer per queried row.

Make password hashing fast (vapor#189)