simplify some functions

* use `OpenSSL::BN` functionality to speed up and simplify `ssh_public_key_conversion`
* same for `unpacked_byte_array` (which has to be *exceptionally* convoluted to avoid api breakage)
* use `Base64::strict_encoding` and `Base64::strict_decoding` to avoid the `gsub` call
* delete obsolete functions and a test for one of them

Potentially controversial:
`unpacked_byte_array` returns an array of `OpenSSL::BN` objects instead of `Bigint`. There were no tests requiring the previous behavior, so I felt free to change it - the function is private anyway and `OpenSSL::BN` is more useful in this case.