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/ach/.gitignore

@@ -4,3 +4,4 @@
 functions
 simple
 externs
+units

users/glittershark/achilles/ach/units.ach

@@ -0,0 +1,7 @@
+extern puts : fn cstring -> int
+
+ty print : fn cstring -> ()
+fn print x = let _ = puts x in ()
+
+ty main : fn -> int
+fn main = let _ = print "hi" in 0