Fix counting of the local variables

The code assumed that the number of `Local` and number of locals is the
same thing. In reality though it is not. `Local` actually represents a
group of locals with the same type. The group can declare more than one
variable and the number of them is returned by `Local::count`.

In this PR we acknowledge this fact. Along the way we add a checked
arithmetic for locals_count and max_stack_height summation.

tests/diff.rs

@@ -93,6 +93,7 @@ mod stack_height {
 	def_stack_height_test!(table);
 	def_stack_height_test!(global);
 	def_stack_height_test!(imports);
+	def_stack_height_test!(many_locals);
 }
 
 mod gas {