Improved support for nkSymChoices in type coercions

For example, this allows you to pick up a proc with a specific signature from
an overload set.

bugfix: nimrod generated invalid code when a RVO function had a single
compile-time param

bugfix: nkHiddenDerefs were not neutralised properly for indirect proc calls

compiler/ast.nim

@@ -715,6 +715,7 @@ const
                   nkCommand, nkCallStrLit}
 
   nkLambdaKinds* = {nkLambda, nkDo}
+  nkSymChoices* = {nkClosedSymChoice, nkOpenSymChoice}
 
   skLocalVars* = {skVar, skLet, skForVar, skParam}