collect all Fuchsia bindings into the `fuchsia` module
The Fuchsia bindings are currently spread out across multiple modules in `sys/pal/unix` leading to unnecessary duplication. This PR moves all of these definitions into `sys::pal::unix::fuchsia` and additionally:
* deduplicates the definitions
* makes the error names consistent
* marks `zx_thread_self` and `zx_clock_get_monotonic` as safe extern functions
* removes unused items (there's no need to maintain these bindings if we're not going to use them)
* removes the documentation for the definitions (contributors should always consult the platform documentation, duplicating that here is just an extra maintenance burden)
`@rustbot` ping fuchsia
fe97fe45f8