Be precise about the stack budget

[brson]

The calculations involved in deciding when the stack has run out are very, very imprecise. User code should be allowed to take up exactly the amount the task is configured for. This will involve discounting the red zone and any unused space before the red zone, and also ensuring that the final stack segment is configured such that the stack limit is set to the appropriate address to enforce the stack budget.

[emberian]

Visiting for triage; still relevant. Marking E-hard as it is very non-trivial.

[emberian]

@nikomatsakis @brson with the new runtime/stack work is this still relevant?

[brson]

I guess this is kind of obsolete since the code this was referencing no longer exists.