Add Rmpfr_rec_root, provided by Vincent Lefevre: (See https://sympa.inria.fr/sympa/arc/mpfr/2016-12/msg00032.html) Re-arranged as an XSub, and some special case handling added.
Add Rmpfr_beta (mpfr-4.0.0 and later only).
Rename t/new_in_3.2.0.t to t/new_in_4.0.0.t
Add Rmpfr_q_div and Rmpfr_z_div and use them in overload_div to replace the existing code (that was rounding twice).
Add overloading of comparisons with mpq_t and mpz_t
use mpfr library of version of mpfr_get_q (in preference to homegrown rendition) iff MPFR_VERSION_MAJOR >= 4.
Workaround mpfr_get_float128() failure to round subnormals reliably.