refactorings to prepare the compiler for IC (#15935)

* added ic specific Nim code; WIP
* make the symbol import mechanism lazy; WIP
* ensure that modules can be imported multiple times
* ambiguity checking
* handle converters and TR macros properly
* make 'enum' test category green again
* special logic for semi-pure enums
* makes nimsuggest tests green again
* fixes nimdata
* makes nimpy green again
* makes more important packages work

lib/system.nim

@@ -627,7 +627,7 @@ proc newSeq*[T](s: var seq[T], len: Natural) {.magic: "NewSeq", noSideEffect.}
   ## the sequence instead of adding them. Example:
   ##
   ## .. code-block:: Nim
-  ##   var inputStrings : seq[string]
+  ##   var inputStrings: seq[string]
   ##   newSeq(inputStrings, 3)
   ##   assert len(inputStrings) == 3
   ##   inputStrings[0] = "The fourth"