I observed that the list of restricted words didn't cover some
attractive words when I attempted to generate a Rust client for
a schema that used keywords like `async` as field names.

- Extended the list of names in `is_restricted()`.
- Added a short yaml schema that uses all of the restricted words
  as field names. Put this fixture to work in a test, to ensure no
  regressions.

The list is undoubtedly incomplete, but there is a very clear place
to add new ones (thank you!) and now a test to validate it.

This is the right way to do it.