Linq expansion planning/next steps

[dubiousconst282]

• Infrastructure:

  • [x] ~Introduce a new IR::Lambda intrinsic to represent lambda factories in a single instruction, to make it easier to identify queries~ (this brings too many complications and it's wasteful; detecting queries isn't really difficult and it's easier to just rely on lambda devirtualization instead)
  • [x] General refactor/rewrite
  • [x] Allow single-def vars used inside regions to be SSA renamed (ILImporter.AnalyseVars())

• Support for sources other than arrays:

  • [x] Fallback to IEnumerator
  • [x] Specialized source for Array, List<>, string
  • [x] Specialized source for Enumerable.Range()
  • [ ] Specialize/devirtualize and inline boxed enumerators (ImmutableArray, ArraySegment)

• Operators/stages:

  • Concretization:
  • [x] ToArray
  • [x] ToList
  • [x] ToHashSet
  • [x] ToDictionary
  • Aggregation:
  • [x] Aggregate
    • [x] Support for "unseeded" aggregates (throws if source is empty, somewhat tricky to implement)
  • [x] Count (predicated only)
  • [ ] #23
  • Min, Max (vectorized in .NET 7)
  • Search:
  • [x] Any, All
  • [x] First[OrDefault]
  • [ ] Last[OrDefault] (will probably need some trickery to implement efficiently for random-access sources)
  • [ ] Single[OrDefault]
  • ElementAt[OrDefault] (not profitable)
  • Stages:
  • [x] Select
  • [x] Where
  • [x] Cast, OfType
  • [x] Skip, Take
  • [x] SelectMany
  • Buffering:
    • [ ] Reverse (idea: could have an option to disable buffering for fast reverse)
    • [ ] Order* (this will be tricky since there's no other stable sort function in the BCL)

• Other:

  • [x] Support for loop consumed queries (port for the front-to-back rewrite)
  • [x] #24
  • Otherwise we may leak memory for e.g. File.ReadLines()
  • ~Offer options to preserve behavior (alt: opt-in/out attributes)~
    • List enumerator version/concurrent mod checking
    • Keep null arg checks (really?)

• Known bugs:

  • [x] Overlapping queries (.ToArray().ToArray())