* invalid visible string support

this allows utf8 in visiblestring, which is not valid DER. we raise a
warning when this happens, but allow it since belgian eIDs, among
others, have encoding errors. Belgium fixed this by 2021 (and possibly
earlier), but their eID certificates have 10 year validity.

* review comments

* clippy

This keeps the buffer object alive, in addition to the original object. Some buffer-implementors have different behavior based on whether there's a buffer object alive.

This is extra mega cursed, and strictly speaking unsound. It does, however, match the status quo ante, where someone mutating a buffer while its being used in cffi code will basically always be UB.

* remove verify_interface

* ruff ruff

* deprecate support for DSA in load_ssh_public_key

* try to prevent bad things a bit more

this will help us revisit it in the future