Don't switch to the C stack when there is room on the Rust stack

[brson]

If there is sufficient room on the Rust stack to run a particular foreign function then we don't need to switch stacks. Each scheduler has a native stack size so we just need the Rust segment to be larger than that (and they can be much larger).

At that point we can annotate specific functions in core with their required stack size. It could significantly reduce the number of stack switches we do.

[brson]

Related to #4479

[graydon]

@pcwalton is this in-scope or related to what you're doing in 0.7?

[graydon]

assigning bug; change assignment if you disagree

[brson]

Not going to make 0.7

[bblum]

I remember thinking about this case a bit during my starter project last summer. This is definitely a feasible optimization, but you have to be careful(?) in the case where C calls back to Rust (what was once charmingly called "crust" functions).

nominating production-ready

[graydon]

subsumed in #3678