[RFC] MPFR and MPC on Base

Hello,

as was discussed in #2564, I doing this pull request with a tentative integration of both MPFR and MPC on Base. It is currently working and passes all tests (I used the same tests I wrote when they were in packages), so it's easy for anyone to try it.

### Known issues:
- [ ] `make check` for MPFR and MPC runs their test suites, lengthening considerably the build process, as `make check` is always called, and I have yet to discover how to bypass this.
- [ ] There is no written documentation currently, nor examples other than the tests.
- [ ] Support for different rounding modes is yet to be implemented (my plans currently include using blocks like used for the precision, and maybe keyword arguments).