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/fixtures/stack-height/many_locals.wat

@@ -0,0 +1,10 @@
+(module
+	(func $one-group-many-locals
+		(local i64) (local i64) (local i32)
+	)
+	(func $main
+		(call
+			$one-group-many-locals
+		)
+  	)
+)