Introduce `ca_types.h`, `gr_types.h` and `nf_types.h`

[albinahlback]

Perhaps these headers should be split in some other ways. For instance, I pushed qqbar_t into ca_types.h, which perhaps is not the most fitting.

• Converted WARN_UNUSED_RESULT to FLINT_WARN_UNUSED.
• Reduced a lot of inclusions in headers.

A multithreaded build now takes around 2% less time to build than before.

Will squash when it seems to be okay.

[fredrik-johansson]

Looks generally OK, but not sure about moving the qqbar and fexpr types which are really independent of the ca types.

[fredrik-johansson]

Also, nf_elem.h and nf.h aren't big files, nor widely included in other files. We could just as well keep the types there and de-inline the remaining inline functions in nf_elem.h to save on some bloat, I think.

[albinahlback]

Yeah, I agree. Perhaps group together the algebraic number theory stuff in some nice way, namely nf, qfb and qqbar?

[albinahlback]

Superseded by #1987