Math-MPFR

Changes for version 4.13

Fix bug in t/rndna.t and t/rndna2.t (exposed by: http://www.cpantesters.org/cpan/report/90030d18-8f4f-11e9-a14c-c69b77725194 )
Change stringification of 'inf' & 'nan' from '@Inf@' & '@NaN@'to 'Inf' & 'NaN, except for Rmpfr_deref2(), which will still retain the actual inf or nan string returned by mpfr - ie '@Inf@' & '@NaN@'.
Remove definition of MAXIMUM_ALLOWABLE_BASE as it is always 62 now that we no longer support versions of MPFR older than 3.0.0.
Accommodate extended range of allowable bases for mpfr_out_str with MPFR-4.1.0 and later.
Cleanup of some code called by nvtoa().
Fix bug in _get_exp_and_bits() for DoubleDoubles. Integer values > 2 ** 105 were being misrepresented by nvtoa().
Improve the private functions _d_bytes, _ld_bytes, _dd_bytes, _f128_bytes
Replace the private functions _d_bytes_fr, _ld_bytes_fr, _dd_bytes_fr, _f128_bytes_fr with the single function _bytes_fr.
Remove mpfr_min_inter_base() and mpfr_max_orig_base() as both are of no value.

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