Move the stack layout computation into its own module.

This is trying to keep algorithms out if the ir module which deals with
the intermediate representation.

Also give the layout_stack() function a Result return value so it can
report a soft error when the stack frame is too large instead of
asserting. Since local variables can be arbitrarily large, it is easy
enough to overflow the stack with even a small function.

lib/cretonne/src/result.rs

@@ -7,7 +7,7 @@ use std::fmt;
 /// A compilation error.
 ///
 /// When Cretonne fails to compile a function, it will return one of these error codes.
-#[derive(Debug)]
+#[derive(Debug, PartialEq, Eq)]
 pub enum CtonError {
     /// An IL verifier error.
     ///