bidirectional-map | Create key/value collections of one-to-one

I'm currently trying to implement base64 encoding/decoding in rust as part of a larger project and needed a data structure for mapping the base64 character set to ASCII and vice versa. I found a barebones implementation of a bidirectional HashMap in this StackOverflow answer, but since I need the base64 map to be a global variable in order for both encode/decode functions to access it, I can't just add the character mappings one by one. So I added another function to the BidiMap implementation that does it in one batch (other function implementations are the same as in the link above):