UUID - Data type inefficient

The new data type, UUID, is stored as a string -char(16)-:

------------
struct pg_uuid_t
{
unsigned char data[UUID_LEN];
};
#define UUID_LEN 16
------------

but this it's very inefficient as you can read here [1].

The ideal would be use bit(128), but today isn't possible. One
possible solution would be create a structure with 2 fields, each one
with bit(64).

[1] http://www.mysqlperformanceblog.com/2007/03/13/to-uuid-or-not-to-uuid/