Better source mapping for Stylus

[tonistiigi]

Styler does CSS to Stylus source mapping by parsing with custom parse step on compilation. This is wrong and very limited. It only works on single file scope and (usually) doesn't figure out the mixin stuff.

Instead these refereneces should be kept and written to the separate stack in evaluator part.

Changes should be in the Stylus code base so other projects could also benefit from it.