* ADD new type, protover::UnknownProtocol, so that we have greater type safety
and our protover functionality which works with unsanitised protocol names is
more clearly demarcated.
* REFACTOR protover::Proto, renaming it protover::Protocol to mirror the new
protover::UnknownProtocol type name.
* ADD a utility conversion of `impl From<Protocol> for UnknownProtocol` so that
we can easily with known protocols and unknown protocols simultaneously
(e.g. doing comparisons, checking their version numbers), while not allowing
UnknownProtocols to be accidentally used in functions which should only take
Protocols.
* FIXES part of #24031: https://bugs.torproject.org/24031
Rationale: this helps for performance only, but we don't actually
have any reason to think that the checks here are
performance-critical. Let's not normalize the use of unsafe {}.
* FIXES#25127: https://bugs.torproject.org/25127
* ADDS a new module to the Rust tor_util crate for small utilities
for working with static strings between languages.
* CHANGES the return type of protover_compute_for_old_tor to point to
immutable data.
* CHANGES the code from the previous commit to use the new static
string utilities.
