change and rename gmp_string_asprintf to return an std::string

change and rename gmp_string_asprintf to return an std::string

This was suggested by Simon during a code review of this package upstream.
The upside is that this makes the function's API more natural and C++.
The downside is an extra malloc, which might be the reason why we went
for using a unique_xmalloc_ptr in the first place. Since this function
is not expected to be called frequently, the API improvement might be
worth the performance impact.

gdb/ChangeLog:

        * gmp-utils.h (gmp_string_printf): Rename from gmp_string_asprintf.
        Change return type to std::string. Update all callers.
        * gmp-utils.c (gmp_string_printf): Likewise.