This will allow us to do actual error handling intra-crate in a more
rusty manner, e.g. propogating errors in match statements, conversion
between error types, logging messages, etc.
* FIXES part of #24031: https://bugs.torproject.org/24031.
Also correct MAX_VERSIONS_TO_EXPAND to match the C.
NOTE that this patch leads to incorrect behavior: the C code allows
huge ranges; it just doesn't allow votes on them (currently). For
full compatibility, we'll need to make the rust code store ranges as
ranges natively, possibly using something like the range_map crate.
Still, this patch is smaller than a "proper" fix.
Fixes TROVE-2018-003.
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.
And fix the unsupported protover example so it uses a Link protover much
higher than 5.
Part of #25070, bugfix on 0.3.3.1-alpha, which introduced the protover crate.
