Representation of BLD lettering schemes

[lgarron]

A request from Bernhard Brodowsky.

https://www.speedsolving.com/wiki/index.php/Speffz is the de facto interoperable standard, but many people still use their own scheme and it would be useful to provide a standard spec (ideally a simple string) to interoperate.

My first thought is to replace the letters in Reid notation with sticker names, but that may not be sufficient for larger cubes (e.g. if people don't orbits like Speffz).