groupByMap should have SameValueZero semantics (#18)

* groupByMap should have SameValueZero semantics

* convert to -0 to +0

Author: bakkot

spec.emu

@@ -86,14 +86,15 @@ location: https://tc39.es/proposal-array-grouping/
       1. Repeat, while _k_ < _len_
         1. Let _Pk_ be ! ToString(𝔽(_k_)).
         1. Let _kValue_ be ? Get(_O_, _Pk_).
-        1. Let _propertyKey_ be ? ToPropertyKey(? Call(_callbackfn_, _thisArg_, « _kValue_, 𝔽(_k_), _O_ »)).
+        1. Let _key_ be ? Call(_callbackfn_, _thisArg_, « _kValue_, 𝔽(_k_), _O_ »).
+        1. If _key_ is *-0*<sub>𝔽</sub>, set _key_ to *+0*<sub>𝔽</sub>.
         1. Let _group_ be ~empty~.
         1. For each Record { [[Key]], [[Elements]] } _g_ of _groups_, do
-          1. If ! SameValueNonNumeric(_g_.[[Key]], _propertyKey_) is *true*, then
+          1. If ! SameValueZero(_g_.[[Key]], _key_) is *true*, then
             1. Set _group_ to _g_.
         1. If _group_ is ~empty~, then
           1. Let _elements_ be a new empty List.
-          1. Set _group_ to the Record { [[Key]]: _propertyKey_, [[Elements]]: _elements_ }.
+          1. Set _group_ to the Record { [[Key]]: _key_, [[Elements]]: _elements_ }.
           1. Append _group_ as the last element of _groups_.
         1. Append _kValue_ as the last element of _group_.[[Elements]].
         1. Set _k_ to _k_ + 1.