* Delete several blocks of unused code.
Delete several pieces of code from libc-bottom-half/cloudlibc that aren't in
use on wasi-libc.
* Delete more of `_CLOCK_PROCESS_CPUTIME_ID` or `_CLOCK_THREAD_CPUTIME_ID`.
* Make __wasi_linkcount_t a uint64_t (#134)
Refs: https://github.com/WebAssembly/WASI/pull/127
* Generate the WASI interface from witx.
This replaces the hand-maintained <wasi/core.h> header with a
<wasi/api.h> generated from witx.
Most of the churn here is caused by upstream WASI renamings; hopefully
in the future ABI updates will be less noisy.
