General Fusion value interning

[toddjonker]

We have need for an intern table that's not limited to symbols. Racket interns a number of kinds of values, and Fusion already has (at least) three intern tables:

• Unannotated symbols
• Annotated symbols
• ModuleIdentitys

Since symbols are so performance-critical, my instinct is to keep those separate. I don't know a reason to have any further partitions though, so the other two should be combined into a common global intern table.