words, syllables, letters and their inverse for fpgroup and pc group elems in all variations

[fingolfin]

Just talked with @fieker: we have syllables(g::Union{FPGroupElem, SubFPGroupElem}) and letters(g::FPGroupElem)

1. [ ] both of these should be available for FPGroupElem and SubFPGroupElem
2. [ ] but ideally also both for PcGroupElem and SubPcGroupElem (as a subcase of the old issue #952)
3. [ ] I just noticed there is also a nice WeylGroup type with a word method (cc @lgoettgens) -- it would make sense to have syllables and letters for those, too
4. [ ] conversely, there is a word method for WeylGroup which looks similar to letters (except it returns a Vector{UInt8} -- of course if the generators are involutions one doesn't need inverses. Anyway: perhaps there should also be a word method for fp and pc (sub) group elems (to be decided what it should return
5. [ ] in all these case, we also want a "reverse" function that takes the output of one of these function, plus a parent (sub)group, and re-constructs a group element.
6. [ ] ... and the docstrings for these functions should always also mention their "inverse" (and ideally include an example showing a roundtrip)

@ThomasBreuer perhaps you can have a look?