🧪 Add evaluation tests to ensure that passes retain NGR semantics. (#2)

This change adds `Arbitrary` instances to the key IR data types (both as syntax and as native IR), as well as evaluator functions for both. This way, we can ensure that the evaluation of one version of the IR is observationally equivalent to another version of the IR, or even a later IR. It also adds a similar ability through both static file compilation and the JIT, to ensure that the translation through Cranelift and our runtime works as expected.

This actually found a couple issues in its creation, and I hope is helpful extensions into more interesting programs.

src/ir.rs

@@ -1,4 +1,5 @@
 mod ast;
+mod eval;
 mod from_syntax;
 mod strings;