feat(gs/achilles): Implement very basic monomorphization

Implement very basic monomorphization, by recording type variable
instantiations when typechecking Call nodes and then using those in a
new hir Visitor trait to copy the body of any generic decls for each
possible set of instantiation of the type variables.

Change-Id: Iab54030973e5d66e2b8bcd074b4cb6c001a90123
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2617
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/glittershark/achilles/src/main.rs

@@ -6,6 +6,7 @@ pub(crate) mod commands;
 pub(crate) mod common;
 pub mod compiler;
 pub mod interpreter;
+pub(crate) mod passes;
 #[macro_use]
 pub mod parser;
 pub mod tc;