[RFC] MPFR and MPC on Base

Hello,

as it was discussed in #2564, I am 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.
- [x] Support for different rounding modes.
- [x] Renaming `MPFRFloat` to `BigFloat`.
- [x] Renaming precision-related functions
- [x] `convert` methods for `BigComplex`
- Migration to a direct representation of the structs involved, instead of a bunch of memory allocated as an array
  - [x] `BigInt`
  - [x] `BigFloat`
- [x] `mpfr_hypot`
- [x] Better basic operations rules between BigTypes and primitives to avoid using promotion to deal with it
- [x] Set 256 as the default precision.
- [x] Add the needed `-dev` libraries to `.travis.yml`
- [ ] Reimplement `sum` for `MPFRFloat` in a way that works, as it needs an array of pointers.
- [ ] MPFR has no `make distclean` option, it seems.