Add lower-level functionality to manage file descriptors directly to the ``C_FileSystem`` API

[rdw-software]

Leftover from #39. I don't need this right now, but it would be nice to have.

Proposed API: Open(path), Close(fileDescriptor), Read(fileDescriptor), Write(fileDescriptor, data); possibly also Append(fileDescriptor, data), Truncate(fileDescriptor)

Can use fs builtin: open, close, read, write