zerocopy: move repo to zerocopy 0.8

[mattkur]

This is a massive commit. Thankfully, much of the change is mechanical.
This change leaves some classes of improvements & fix-ups:

• map_err: These are cases where an error was already returned. Now that
  zerocopy 0.8 methods return a Result<...> rather than an Option<...>,
  those errors could be enriched with the details returned from the zerocopy
  methods.
• err, option-to-err, unwrap, etc.: These are cases where an error is
  turned into a panic or None (Option). In these cases, the calling code
  could better leverage the zerocopy error handling.
• review carefully: this is due to some cases where the changes are
  less mechanical, so should be treated as suspect.
• use-rest-of-range: these are cases where the zerocopy support
  functions now return a tuple (T, &[u8]), where the slice is the
  rest of the buffer. In some cases, the code calling the zerocopy
  methods could use the resulting slice rather than walking
  the data type manually.

Fixes #313