nr2.0: Add ForeverStack data structure.

This data structure replaces our existing scopes and allows for multiple
name resolution passes to insert definitions or look up names at different
stages of the pipeline. The documentation goes into detail about how the
data structure works, and how the source's hierarchy is preserved
throughout the pipeline.

The class is templated to enforce specific behavior based on the namespace
the scope map targets.

gcc/rust/ChangeLog:

	* resolve/rust-forever-stack.h: New file.
	* resolve/rust-forever-stack.hxx: New file.