Support running WASM+WASI binaries via the wasmi interpreter (#472)

* See `applications/wasm` for examples on how to run WASM binaries.

* Key components and details:
    * `kernel/wasm_interpreter` is the main interface between `wasmi` and the rest of Theseus.
    * `applications/wasm` is the user frontend for invoking a WASM binary, which allows specifying access to directories, preopening files, and passing arguments to the WASM application.
    * Basic WASI system calls are supported.
    *  WASM binaries are currently located in `extra_files/`, as are some test/demo files that are used by those WASM binaries. f4aa715