unknown symbolic values by renaming some diagnostics and
creating new unknown reasons for each type of failure that
can happen during constant evaluation.
Adds memory objects and addresses to the constant interpreter, and
teaches the constant interpreter to interpret various instructions that
deal with memory and addresses.
Implements a constant interpreter that can deal with basic integer operations.
Summary of the features that it includes:
* builtin integer values, and builtin integer insts
* struct and tuple values, and insts that construct and extract them (necessary to use stdlib integers)
* function referencing and application (necessary to call stdlib integer functions)
* error handling data structures and logic, for telling you why your value is not evaluatable
* metatype values (not necessary for integers, but it's only a few extra lines, so I thought it would be more trouble than it's worth to put them in a separate PR)
* conditional branches (ditto)
