introduce iterable[T] type for procs like toSeq

[timotheecour]

goal: introduce a type for called iterators instead of being treated as untyped (which will make elementType and toSeq well typed), which would resolve some other issues:
nim-lang/RFCs#512
nim-lang#9219
nim-lang#13595
nim-lang#14148
nim-lang#16545
and sidestep nim-lang#14778
and reduce cases where we hit nim-lang#14827

Note: concepts aren't powerful enough to express that, so type Iterable[T] = concept can't work

links

related

• every symbol becomes 1st class; defines 0-cost lambda and aliases; generics/iterators/templates/etc can be passed to any routine nim-lang/Nim#11992 (aliassym, making symbols, iterators and other symbols 1st class)
• enableif: simpler and more powerful alternative to concepts nim-lang/Nim#12048 (enableif, allowing to express arbitrary constraints in routine signatures)
  I've implemnted this in fact, see every symbol becomes 1st class; defines 0-cost lambda and aliases; generics/iterators/templates/etc can be passed to any routine nim-lang/Nim#11992 (comment)

implementation

=> PR nim-lang#17196