[WIP] Specialize Where.First, Where.Last and the First and Last overloads accepting predicates

[jamesqo]

Changes:

• source.Where(predicate).First() should have the same result as source.First(predicate) (barring exceptions), so if we optimize for one we can optimize for the other. This PR speeds things up in both cases if source is an array/List/IList, avoiding double interface dispatch and an enumerator allocation.

• source.Where(predicate).Last() is now recognized in addition to source.Last(predicate), so we don't run the predicate on every item if source supports random access.

• I also added code to specialize ilist.Where(predicate). It makes sense because we can check for IList at little cost to existing codepaths, and there is already logic in place for ilist.First(predicate) and ilist.Last(predicate), so we can implement ilist.Where(predicate).First() and ilist.Where(predicate).Last() for free.

Marking WIP temporarily because I need to finish adding XML docs and make sure all branches are being covered. Comments are welcome in the meantime though.

/cc @JonHanna @VSadov

edit: Forgot the most important thing-- performance results. Here they are. There seems to be an improvement across the board for basically every pattern. The leaked memory stats can be ignored, I added that as part of #15389.

[jamesqo]

Closing temporarily. I haven't gotten around to making progress on this so far.