wasi-threads vs wasm start function

[yamt]

our wasi-threads implementation calls instance_create within a host function.
depending on what the start function does, it can be problematic.
maybe it's simpler to leave the instance_create_execute_init part to the spawned thread.

• spec-wise, on which thread the start function is supposed to be called?
• can the start function perform problematic operations? eg. acquire a lock
• if the start function traps, what should happen? should thread-spawn fail?
• should wasi:thread-spawn synchronously create a new instance?