Better support for treating templates and macros as symbols.

This allows you to pass a template or a macro to another macro
which can then inspect the implementation of the former template/macro
using `getImpl`.

Since templates can be freely redefined, this allows you to treat
their symbols as compile-time variables that have lexical scope.
A motivating PoC example for a logging library taking advantage of
this will be provided in the next commit.

Implementation details:

* The name of a template or a macro will be consider a symbol if
  the template/macro requires parameters

* For parameterless templates/macros, you can use `bindSym`, which
  was extended to also work outside of compile-time procs.

compiler/semmagic.nim

@@ -199,6 +199,10 @@ proc semBindSym(c: PContext, n: PNode): PNode =
   if s != nil:
     # we need to mark all symbols:
     var sc = symChoice(c, id, s, TSymChoiceRule(isMixin.intVal))
+    if not getCurrOwner(c).isCompileTimeProc:
+      # inside regular code, bindSym resolves to the sym-choice
+      # nodes (see tinspectsymbol)
+      return sc
     result.add(sc)
   else:
     errorUndeclaredIdentifier(c, n.sons[1].info, sl.strVal)