Added function for generating keyed alphabet

[dex-r]

This adds a small function for generating an alphabet from a key. A few points:

• I've had to put an allow(dead_code) to prevent a compiler warning, since the function is not yet being used.
• I wasn't entirely sure whether to put the function in its own file. Its currently in common/alphabet.rs but I can move it if you think it would be better on its own.

[arosspope]

Not too keen on adding unused code to the repo. I would much prefer it to be added as part of a PR that uses the function. When that happens:

• I suggest moving it to a seperate file in common called keygen.rs