lang: `If` expression support

Summary

Adds (If <predicate> <body> <alternative>) expression support, as well as gated logic, via the (If <predicated> <body>) variant.

Details

The body only variants are always treated as unit type expressions, while the exhaustive variants can be expressions of any variety so long as the body and alternative branch types match (the same or subtype).

Implemented If, along with a variety of tests for various failure scenarios. Additionally, added the ability to provide extra message info to the unreachable routine, this is useful for debugging.

Notes for Reviews

~I just realized that I can use Return to test the Void cases, so I'll add tests accordingly~
- I can't because we don't support statement lists just yet
I'm not too sure about the quality of error messages, but maybe that doesn't matter?
I still don't have a good feel for code style as I'm getting used to the codebase

nim-works / phy