Spill/reload storage in the stack frame

It's all in commit 0b9cf2f3049732de72ef4801f25b58b29ba73482 ("Support for spill/reload storage in stack frame"). I still need to convert the back-end to use it so I can actually generate spill/reload instructions. I am working towards live interval splitting bottom-up now and I am almost there. Maybe in few weeks I can finally add support for callee/caller saved registers that fixes the currently broken test...