Wasmtime: lift fiber creation implementation into instance allocator

[peterhuene]

Once #2518 lands, we should lift the fiber creation implementation out of the wasmtime-fiber crate and into the instance allocator implementation.

This will fix the bit wonky "not supported" error the instance allocator returns to indicate the fiber crate should create its own fiber.

[peterhuene]

In addition to this, we should add a test case that hits a fiber stack guard page and ensures a crash with both the on-demand and pooling instance allocators.