gmp-utils: New API to simply use of GMP's integer/rational/float objects

gmp-utils: New API to simply use of GMP's integer/rational/float objects

This API was motivated by a number of reasons:
  - GMP's API does not handle "long long" and "unsigned long long",
    so using LONGEST and ULONGEST is not straightforward;
  - Automate the need to initialize GMP objects before use, and
    clear them when no longer used.

However, this API grew also to help with similar matter such
as formatting to a string, and also reading/writing fixed-point
values from byte buffers.

Dedicated unit testing is also added.

gdb/ChangeLog:

        * gmp-utils.h,  gmp-utils.h: New file.
        * unittests/gmp-utils-selftests.c: New file.
        * Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
        unittests/gmp-utils-selftests.c.
        (COMMON_SFILES) Add gmp-utils.c.
        (HFILES_NO_SRCDIR): Add gmp-utils.h.