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.
* No need for mem::replace when doing a simple assignment
* Allow specifying the module of the imported 'gas' function
This allows users to place the imported function inside a custom
module instead of the generic 'env' module.
Sergey Shulepov