Add `bit_array.to_string_utf16` and `to_string_utf32`

Some binary protocols (for example [ID3](https://id3.org/)) use UTF-16 encoded text, which right now must be manually converted to a string. It could be useful to have `bit_array.to_string_utf16` function to perform this conversion. `to_string_utf32` is not something that I've personally needed, but could be useful as well.

Additionally it could be helpful to be able to specify endianness as that is something which varies across standards.