gmp-utils: Convert the read/write methods to using gdb::array_view

gmp-utils: Convert the read/write methods to using gdb::array_view

This commit changes the interfaces of some of the methods declared
in gmp-utils to take a gdb::array_view of gdb_byte instead of a
(gdb_byte *, size) couple.

This makes these methods' API probably more C++-idiomatic.

        * gmp-utils.h (gdb_mpz::read): Change buf and len parameters
        into one single gdb::array_view parameter.
        (gdb_mpz::write): Likewise.
        (gdb_mpq::read_fixed_point, gdb_mpq::write_fixed_point): Likewise.
        * gmp-utils.c (gdb_mpz::read): Change buf and len parameters
        into one single gdb::array_view parameter.
        Adjust implementation accordingly.
        (gdb_mpz::write): Likewise.
        (gdb_mpq::read_fixed_point, gdb_mpq::write_fixed_point): Likewise.
        * unittests/gmp-utils-selftests.c: Adapt following changes above.
        * valarith.c, valops.c, valprint.c, value.c: Likewise.