#24 OnBC: Better switch statement codegen

[Steve Little]

As per Philippe Guillot's email:

"The switch statement generates an additionnal local
variable on the stack. I my opinion, this is useless. It
would be better to keep the value to be compared in the
D0 register (smaller and more efficient code and probably
simpler in the compiler)."

Again, I've not assessed the feasibility of this yet.