rewriter: support the 6 x86 param regs for post condition functions

[kkysen]

This is done by pushing the param regs onto the stack in the prologue (if there's a post condition function), and then popping them before we call the post condition function.

Next we'll work on supporting more than the 6 param regs, as well as passing the return value.

[ayrtonm]

Next we'll work on supporting more than the 6 param regs, as well as passing the return value

Before either of these things I think we should explore what the checks themselves will look like. So trying out ways of allowing a developer to opt-in to simple checks or providing a way to specify their own more specific checks.

In general I'd like us to have a more clear picture of what this looks like/maybe have an "end-to-end" demo or test before fixing the details like passing in arguments correctly for all possible function signatures. The same goes for return values although we may want to handle that earlier to support checks that are conditional on the return value.