Bring segmented stack back to embedded targets

[matklad]

This discussion seems related:

What is the current safety story for library-based stackful coroutines?

One interesting reslult of the last round of tech empower benchmarks is that one of the top slots is occupied by may-minihttp: may is a stackful coroutine library -- it uses asm tricks to implement cooperative scheduling of green threads. However, may is not safe to use -- spawning a coroutine is an unsafe operation, because acessing TLS from a couroutine can lead to undefined behavior (and not just errors): may/coroutine_impl.rs at 5e1fe6392360267c092e7c5879ab96e5d68fc28d · Xudong-Huang/may…

It would be cool if Rust were low-level enough language to let stack allocation strategy be implemented in the library.