Refactor: libcrmcommon: Replace buffer_print() with GString

buffer_print() is better than manually dealing with offsets and maxes
every time we need to append data to a string buffer, but it's still
cumbersome. We have to keep track of offset and max with every call.

Here we replace it with calls to GLib's string functions.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>