Handling the exit trap

[whitequark]

Since wasmtime-py 0.18.0, it seems that when the main() function returns, the call to _start() also simply returns, which is good. However, it looks like if exit() is called, then the result is a wasmtime._trap.Trap: Exited with i32 exit status 1. It is not clear how this trap can be handled besides matching the exception message with a regex.

[whitequark]

It looks like I am mistaken; the only change in behavior is that nonzero exit codes result in a trap now. That's great, actually! But I'd still need some nice way to handle the trap.

[whitequark]

After more testing I think my original conclusion was accurate; I later dismissed it because wasmtime 0.17 had another issue, which is that it looks like setting argv doesn't work at all.

[alexcrichton]

Ah yes, I'll add C API functions for inspecting this so a new exception class can be implemented here.

[alexcrichton]

Ok I've added bindings for this at #35