Proposal: remove MPFR dependency

[tkfu]

As far as I can see, the only truly required use of MPFR is for correct rounding towards zero via mpfr_get_z (ret->bytes, size, MPFR_RNDZ). Would you consider a patch that removed MPFR, and just added a helper function for rounding? (And then switch the rest of the uses of MPFR to use the equivalent mpf_ functions from GMP.) I've got a local version of this patch that's passing tests.

My issue is that MPFR is GPLv3 only since quite some time now, and this ends up being a blocker for embedded systems that will use secureboot, because of the GPLv3 anti-tivoization clause.

[tkfu]

Most of the discussion of this happened on #161, but just for posterity: It turns out there's a 2nd good reason to remove the MPFR dependency. It's true that using MPFR made libbytesize correctly parse '0.1 KB' as 100 bytes instead of 99. But it didn't solve the problem in the general case, it just made those kind of off-by-one errors due to floating point precision problems less common. After testing with a fuzzer, I made an implementation in #163 that uses libgmp rationals instead of floats, and added regression tests for cases that fail in both types of float (libgmp and MPFR). As a side effect, this removes the MPFR dependency.