gen-headers: Generate assertions of layout from witx (#149)

* use pch/layout branch for witx; generate assertions of layout

* address review comments, add asserts for handle

* change wasm32 support comment to a preprocessor error

* expose `to_c_header` in wasi-headers crate for use in external test harness

* main.rs: inputs and output arguments are optional

so that generate-libc command works

* regen header

Author: pchickey