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

changelog.md

@@ -302,6 +302,10 @@
 - `nim e` now accepts arbitrary file extensions for the nimscript file,
   although `.nims` is still the preferred extension in general.
 
+- Added `iterable[T]` type class to match called iterators, which enables writing:
+  `template fn(a: iterable)` instead of `template fn(a: untyped)`
+
+
 ## Compiler changes
 
 - Added `--declaredlocs` to show symbol declaration location in messages.
@@ -341,6 +345,7 @@
 - `--hint:CC` now goes to stderr (like all other hints) instead of stdout.
 
 
+
 ## Tool changes
 
 - The rst parser now supports markdown table syntax.