Design compiler IR

[dtolnay]

The current IR (based on WipFunction for functions, and ValueRef + ValueNode for values) just barely works for simple macros but is probably not going to be pleasant to work with internally as the compiler gets more interesting.

Basically the entire current implementation in the src/ directory is just supposed to be a proof of concept that we can stack enough RefCells on top of each other to get something reasonably convenient from the user's perspective. 😈 I am open to completely replacing it as needed.