import MPFR 4.2.1.

mostly a bug-fix release, highlights include:
- abort on lock failure, instead of just warn
- better Inf handling
- fix an unlikely stack overflow in mpfr_rec_sqrt()
- fixes for mpfr_reldiff()
- fix boundary error in mpfr_pow_general()
- fixes to printing Nan and Inf
- many manual and test updates

initial import of MPFR 4.2.0. changes from 4.1.0 include:

Binary compatible with MPFR 4.0.* and 4.1.*, though some minor changes
in the behavior of the formatted output functions may be visible,
regarded as underspecified behavior or bug fixes (see below).

New functions mpfr_cosu, mpfr_sinu, mpfr_tanu, mpfr_acosu, mpfr_asinu,
mpfr_atanu and mpfr_atan2u.

New functions mpfr_cospi, mpfr_sinpi, mpfr_tanpi, mpfr_acospi,
mpfr_asinpi, mpfr_atanpi and mpfr_atan2pi.

New functions mpfr_log2p1, mpfr_log10p1, mpfr_exp2m1, mpfr_exp10m1 and
mpfr_compound_si.

New functions mpfr_fmod_ui, mpfr_powr, mpfr_pown, mpfr_pow_uj,
mpfr_pow_sj and mpfr_rootn_si (mpfr_pown is actually a macro defined as
an alias for mpfr_pow_sj).

Bug fixes.
- In particular, for the formatted output functions (mpfr_printf, etc.),
the case where the precision consists only of a period has been fixed to
be like .0 as specified in the ISO C standard, and the manual has been
corrected and clarified.
- The macros of the custom interface have also been fixed:
they now behave like functions (except a minor limitation for
mpfr_custom_init_set).