provide as_string() method to return the alphabet characters

[JosiahParry]

This PR provides a method to return the characters in the Alphabet struct as a String.

At present, there is no provided way to retrieve characters that were used to create the alphabet. One way to accomplish this today, is to capture the output of the Debug format and process it manually. This as_string() method will make it easier to do just that. Alternatively, if the symbols field of the struct was made public, the onus could be put on the users of the crate to iterate through the array and cast u8 as char.

As an alternative to a GH issue, I wanted to provide a PR instead. Please let me know if this suffices.

[JosiahParry]

Firs time contributing to something with no_std I think i got the cfg correct :P

[marshallpierce]

Thanks for the contribution! I've suggested a simplification (and it looks like you did get the attributes right, but you won't need them after all).

[JosiahParry]

CI Gods are pleased 👼🏼

[marshallpierce]

Released as 0.21.7