initial pass of Tuple group/groupToMap for proposal-array-grouping

[rickbutton]

This PR represents a first pass addition of Tuple.prototype.group and Tuple.prototype.groupToMap, reflecting the addition of similar methods to the Array.prototype in https://github.com/tc39/proposal-array-grouping .

The array grouping proposal is currently at Stage 3, so we don't intend to merge this until Array Grouping reaches Stage 4, but want to prepare this in case it advanced ahead of Records and Tuples.

A few notes on this spec text:

• This is almost entirely the same as the spec text for regular Array versions of these methods, which some verbiage changed to refer to Tuple.
• This relies on the AO AddValueToKeyedGroup that the array grouping proposal implements, so if you are looking for that it is here: https://tc39.es/proposal-array-grouping/#sec-add-value-to-keyed-group
• I removed the comments ahead of the algorithm steps regarding the mutation of the Array (Tuple) being traversed, as they don't apply here, since Tuples are immutable.
• The return value of both of these new methods is an Object of Arrays. This choice was made so that a symbol can be used as a "grouping key" (because a Record cannot have a symbol key, see the discussion in this issue: Tuple.prototype.groupBy #275 (comment))

[nicolo-ribaudo]

The other tuple methods we defined with explicit steps all use thisTupleValue(*this* value) instead of ToObject.

[rickbutton]

makes sense, I will adjust. I wasn't sure how much to deviate from the original Array versions of these methods.

[rickbutton]

Do you think I should instead use the same methodology behind other Array methods like .at and just specify them in terms of the original? Or manually specify the algo here as well.

[ljharb]

manually is probably the better end goal regardless of how you do it in the proposal repo.