Clean up wasi-nn.wit specification (#49)

* Move original documentation to new `wasi-nn.wit` file

This change also removes some extra functions that snuck through the
review of #38: `set-input-by-name`, `eval`.

* Move `wasi-nn.wit` to top-level directory

* Remove original `*.wit.md` documents

* Tweak `get-output` return type

Author: abrown