This commit updates the wasi target with supported added in
CraneStation/wasi-sysroot#10. That function allows both C and Rust to
cooperate in how preopened files are managed, enabling us to learn about
propened files through the same interface. The `open_parent` function in
the wasi `fs` module was updated to avoid its own initialization of a
global preopened map and instead delegate to libc to perform this
functionality.

This should both be more robust into the future in terms of handling
path logic as well as ensuring the propened map is correctly set up at
process boot time. This does currently require some unfortunate
allocations on our side, but if that becomes an issue we can always
paper over those in time!