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/Cargo.toml

@@ -19,6 +19,7 @@ pratt = "0.3.0"
 proptest = "1.0.0"
 test-strategy = "0.1.1"
 thiserror = "1.0.24"
+void = "1.0.2"
 
 [dev-dependencies]
 crate-root = "0.1.3"