Merge: Snake phase names

Default name of phases where based on a lower cased version of their classnames.
That was bad. eg `FFILanguageAssignationPhase` -> `ffilanguageassignation`

So this PR snakecase them instead.

But the spec of `to_snake_case` was awful: `f_f_i_language_assignation`

So also fix the spec of this method and get something nice `ffi_language_assignation`.

Pull-Request: #1276
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>