Add switch to fiber interface, support fiber_switch in asyncify implementation

[yinamy]

This patch:

1. Adds a new interface fiber-switch.h with a new fiber operation fiber_switch.
2. Adds a test program hello_switch.c that uses fiber_switch.
3. Adds an asyncify implementation for fiber_switch which works with the test program above.

[yinamy]

@dhil The new commit tries to do everything you mentioned above, it works with my updated version of hello_switch.c (I'll make some new examples later). I have a few uncertainties about it:

1. In your example code for fiber_main and the main from the user program, you allocate a fiber running the main user program twice. I resolve this by only allocating it in fiber_main, exporting a new function get_active_fiber() in fiber_switch.h so that it can be referenced by the user program, and having it as a global variable in the user program. Not sure if this is the best solution because I feel like the user program shouldn't touch main_fiber this much, but I can't think of anything else for now.
2. What should fibers do when they return? It makes sense to track the previously-running fiber with an extra field in the fiber_t struct (which I added), so that we can just resume the previous fiber when the current one returns. However, my example program keeps switching between the same two fibers hello_fiber and world_fiber as soon as the first switch from main_fiber happens, so main_fiber gets lost. I assume main_fiber should always finish running, so in this commit the user program explicitly tells the last-running user fiber to switch back to main_fiber, and the fiber_main checks if main_fiber is done. Also not sure if this is ideal!

[dhil]

It's looking good. Just a few comments.

Semantically, I think the canonical reduction rule for switch is something like:

[switch] fiber_main(E[fiber_switch(target_E', arg, out)]) ~> fiber_main(E'[(arg, E)])

where E denotes the current evaluation context, and E' subscript denotes the evaluation context of the target, and the notation (arg, E) means substitute arg as the return value in E' and substitute E for out in E'.

[dhil]

I don't understand why this is needed -- I would be hesitant to add it, because I think it will be a bit problematic to implement in the WasmFX backend, as we would need to track which fiber is currently executing there too.

[yinamy]

Would if be better if I changed it to something like get_main_fiber, where it returns fiber_main specifically? (where fiber_main is the fiber running the main function of the user program)