Larger `gid` size in MITCCRH

[djturizo]

The current gate identified gid in MITCCRH is of type uint64_t , meaning that it has 64 bits in length. It would be nice to make gid to be of type block, and therefore having 128 bits of length (equal to the usual security parameter). The change should be painless, and the code already makes the conversion of gid to block before generating the next key. Alternatively, the change could be implemented as a function overload in case some other functions are specifying the value of gid manually.