ENHANCE: Faster rewriting systems for simple groups

- Go thorough pcgroup for word expression iun Borel:
  as this will produce a word in the right ordering, rather than one obtained
  from a stabilizer chain (which will require more expensive rewriting).
- Don't run through all of Borel (but rely on MakeConfluent)
- Faster Borel rewriting (useful for large primes)
- Presentation for rewriting tries to use split generators:
  When getting a presentation for rewriting, try to ensure that the
  generating set chosen exposes direct factors of the radical factor.
  Otherwise there will be a a lot of messy tails which can be costly to
  clean up. (Basically this mimics some of what `SpecialPcgs` does.)
- Stored rewriting systems for some simple groups (up to certain size).