iterable[T] (#17196)

* fix failing test toSeq in manual which now works
* changelog
* reject proc fn(a: iterable)
* add iterable to spec
* remove MCS/UFCS limitation that now works

lib/system.nim

@@ -123,6 +123,10 @@ proc defined*(x: untyped): bool {.magic: "Defined", noSideEffect, compileTime.}
   ##     # Do here programmer friendly expensive sanity checks.
   ##   # Put here the normal code
 
+when defined(nimHasIterable):
+  type
+    iterable*[T] {.magic: IterableType.}  ## Represents an expression that yields `T`
+
 when defined(nimHashOrdinalFixed):
   type
     Ordinal*[T] {.magic: Ordinal.} ## Generic ordinal type. Includes integer,