Instead, we create a single zone-scoped object that's visible for the
entire lifespan of the visitor, and which exposes evaluation internals
which can be updated as direct field modifications.
We now wrap _withStackFrame() around wider sections of code, including
_loadStylesheet() which handles parse errors, so that the @use/@import
stack frames are available.
In addition to being useful for users of Sass, this will make it
possible for core Sass functions to produce warnings without needing
an explicit reference to the evaluator.
