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/lib.rs

@@ -41,5 +41,6 @@ mod partition_slice;
 mod predicates;
 mod ref_slice;
 mod simple_gvn;
+mod stack_layout;
 mod topo_order;
 mod write;