Introduce API for custom stack memory

[rockwotj]

Introduces an API to allow embedders to allocate stacks for fibers using custom memory instead of mmap.

Discussed on Zulip

[rockwotj]

This is a much safer API, because we don't need to assume they will sent env and finalizer to nullptr if they don't need them.

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[fitzgen]

(Going to leave the C API bits to Alex to review)

[fitzgen]

Nice! I looked through the C API changes in more detail as well, and they look good as well.

Thanks @rockwotj!

[rockwotj]

Thanks @fitzgen! I messed up some lint warnings when the async config is disabled. I've fixed them now.