Implement the Trace node from Ruby Sass.

The Trace node is a simple wrapper around the result of a mixin call
and `@content`. It exists as a way to group the resulting properties
as a block but maintain some metadata about the initiating
mixin call or `@content` block.

It also plays an important rule is bubble those blocks with
`@at-root` and nested property sets. This required implementing
the final missing `cssize` methods. As a result we should a complete
`@at-root` implementation.

`Trace` "wrapping" other nodes didn't play nice with the naive check
nesting implementation I lazily wrote so I also implemented 99%
of the check nesting visitor like-for-like with Ruby Sass. The missing
piece is handling `@imports` which is the only part I didn't need to
touch in this work.

Spec sass/sass-spec#868
Fixes #1585

Author: xzyfer

src/expand.cpp

@@ -87,9 +87,6 @@ namespace Sass {
 
   Statement* Expand::operator()(Ruleset* r)
   {
-    bool old_at_root_without_rule = this->at_root_without_rule;
-    // reset when leaving scope
-
     if (in_keyframes) {
       Keyframe_Rule* k = SASS_MEMORY_NEW(ctx.mem, Keyframe_Rule, r->pstate(), r->block()->perform(this)->block());
       if (r->selector()) {
@@ -100,7 +97,8 @@ namespace Sass {
       return k;
     }
 
-    this->at_root_without_rule = false;
+    // reset when leaving scope
+    LOCAL_FLAG(at_root_without_rule, false);
 
     // do some special checks for the base level rules
     if (r->is_root()) {
@@ -154,8 +152,6 @@ namespace Sass {
     rr->is_root(r->is_root());
     rr->tabs(r->tabs());
 
-    this->at_root_without_rule = old_at_root_without_rule;
-
     return rr;
   }