WASI: thread safety

[rdunnington]

The current implementation of the wasi functions are not threadsafe, as wasi file descriptors are backed by an internal data structure that isn't threadsafe, nor wrapped by any locks. This is fine for simple programs, but more complex programs that embed bytebox may invoke wasm functions from different threads. Ideally the solution will also allow compiling thread safety out if it isn't needed to avoid the overhead in single-threaded environments.