Bring group solver up to code standards

[fredrik-bakke]

The group solver recently started to see usage by @lowasser in #1384, so proper naming and formatting of this file is warranted.

[fredrik-bakke]

In particular, it should be possible to make the simplifyExpression function (which should be renamed of course) into a generic $n+2$-ary function using telescope-Level, hence avoiding tuples.

[fredrik-bakke]

We should also write a syntax module (#1332) for group syntax.