sem: correctly type empty containers in macro calls (#1361)

## Summary

Internal-only issue. Empty container values passed to `static` macro/
template parameters used empty container types. Now they're converted
to the formal parameter's type.

## Details

* wrap the arguments to `static` macro/template parameters in
  implicit conversions, if needed for the match
* use post-match argument fitting for macro calls
* for a smaller impact, template calls still don't use post-match
  argument fitting
* remove the `vmserialize` workaround for empty set types (which no
  longer reach there now)

compiler/sem/semexprs.nim

@@ -1182,8 +1182,12 @@ proc afterCallActions(c: PContext; n: PNode, flags: TExprFlags): PNode =
   result = n
   let callee = result[0].sym
   case callee.kind
-  of skMacro: result = semMacroExpr(c, result, callee, flags)
-  of skTemplate: result = semTemplateExpr(c, result, callee, flags)
+  of skMacro:
+    result = fitArgTypesPostMatch(c, result)
+    if result.kind != nkError:
+      result = semMacroExpr(c, result, callee, flags)
+  of skTemplate:
+    result = semTemplateExpr(c, result, callee, flags)
   else:
     semFinishOperands(c, result)
     activate(c, result)

compiler/sem/sigmatch.nim

@@ -2311,10 +2311,26 @@ proc paramTypesMatchAux(m: var TCandidate, f, a: PType,
       of tyTyped, tyTypeDesc:
         arg
       of tyStatic:
-        if arg.typ.n.isNil:  # no value on the type
-          argSemantized
-        else:                # value on the type
-          arg.typ.n
+        let n =
+          if arg.typ.n.isNil:  # no value on the type
+            argSemantized
+          else:                # value on the type
+            arg.typ.n
+
+        # XXX: the implicit conversion handling is duplicated from the non-
+        #      template/non-macro path. Template and macro arguments shouldn't
+        #      be special-cased like this
+        case r
+        of isEqual: n
+        of isGeneric:
+          if n.typ.isEmptyContainer:
+            implicitConv(nkHiddenStdConv, f[0], n, m, c)
+          else:
+            n
+        of isSubtype:
+          implicitConv(nkHiddenSubConv, f[0], n, m, c)
+        else:
+          implicitConv(nkHiddenStdConv, f[0], n, m, c)
       else:
         argSemantized
     return

compiler/vm/vmserialize.nim

@@ -94,11 +94,7 @@ proc initFromExpr(dest: LocHandle, tree: MirTree, n: var int, env: MirEnv,
       # subtract the first element's value to make all values zero-based
       toInt(val - first)
 
-    let first =
-      if tree[n].len > 0: firstOrd(c.config, env[tree[n].typ])
-      else:               Zero
-    # XXX: ^^ ``set[empty]``-typed literals reach here, but they shouldn't. The
-    #      len guard works around the issue
+    let first = firstOrd(c.config, env[tree[n].typ])
     iterTree(j):
       let node = next()
       if node.kind == mnkRange: