feat(achilles): Implement a Unit type

Add support for a zero-sized Unit type. This requires some special at
the codegen level because LLVM (unsurprisingly) only allows Void types
in function return position - to make that a little easier to handle
there's a new pass that strips any unit-only expressions and pulls
unit-only function arguments up to new `let` bindings, so we never have
to actually pass around unit values.

Change-Id: I0fc18a516821f2d69172c42a6a5d246b23471e38
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2695
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/glittershark/achilles/tests/compile.rs

@@ -24,6 +24,11 @@ const FIXTURES: &[Fixture] = &[
         exit_code: 0,
         expected_output: "foobar\n",
     },
+    Fixture {
+        name: "units",
+        exit_code: 0,
+        expected_output: "hi\n",
+    },
 ];
 
 #[test]