Facet types support rewrite (where .A =...) constraints

[josh11b]

• Rewrite constraints are stored in a facet type, substituted, imported, and formatted.
• We now distinguish .Self from other symbolic bindings in two ways:
  • .Self itself now has an invalid compile time binding index (since it doesn't bind to any of the generic parameters). As a result, we no longer need to create a generic region in handle_where.cpp.
  • There is a new phase tracking values that are only symbolic because they transitively depend on .Self. This allows us to give the result of a where expression template phase as long as it doesn't use any symbolic constants other than .Self or other designators.
• AddConstant has been removed from check/context since it was only used from eval. This meant less plumbing of the phase change.
• Evaluation of BindSymbolicName now also performs substitution into its type.
• Include a bit more information in some diagnostics.
• StringifyTypeExpr outputs rewrites, which required adding support for associated entities as well.
  • Associated entities now have an entity name set when importing.
• Adds tests for some interesting cases with rewrites and uses of .Self mixed with other symbolic constants.

Still to do:

• There is no validation that any particular type satisfies rewrite constraints.
• Access to members of a facet type do not see the rewritten values.
• Impls don't recognize whether associated constants have rewrites setting their values.
• No support for resolving facet types.