MINOR: hlua_fcn: add Patref:set()

MINOR: hlua_fcn: add Patref:set()

Just like "set map" on the cli, the Patref:set() method (only relevant
for maps) can be used to modify an existing entry's value in the pattern
reference pointed to by the Lua Patref object. Lookup is performed on the
key. The update will target the live pattern reference version, unless
Patref:prepare() is ongoing.