Restore build of testsuite and benchmarks

[andreasabel]

• Remove trailing whitespace and turn tabs to spaces

• Fix building testsuite (import conflict)

• Make benchmarks build again (fix import conflicts)

Test and benchmarks can however not run in CI due to:

• How to build the testsuite? #68

A hack to build it locally was to rename pretty to something else in the pretty.cabal file.
(Maybe this hack could be institutionalized in the CI using a patch that is applied before the build.)

[andreasabel]

@dterei PR is ready.
Commits should be preserved. There is a larger commit that just fixes whitespace that should not be fused with the other commits.

[andreasabel]

Ok, the build of the testsuite is only finally breaking with GHC 9.8, so I included the testsuite in CI for some older versions of GHC (9.6 for ci.yml, 9.0/2 for Haskell CI).

[dterei]

Thanks @andreasabel! Merged :)