shift read_hex_bytes() and parse_guid_string() into lib/util

shift read_hex_bytes() and parse_guid_string() into lib/util

read_hex_bytes() is going to be used in lib/util/rfc1738.c.

parse_guid_string() is shifted for two reasons: Firstly, it is called
very often in some operations, sometimes constituting a few percent of
the CPU load, and it makes several calls to read_hex_bytes(). We want
the compiler to be able to inline those calls if it thinks that is
wise. Secondly, there are other places that could do with fast GUID
parsing.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>