Rearrange the members of struct _node to put shorter fields towards

the end, in the hope of saving some bytes on 64-bit machines.  (Too
bad n_nchildren can't be made an unsigned short, but
test/test_longexp.py specifically tests for more than 2**16 subtrees
at one level.)

I don't expect any binary compatibility issues here, unless someone
has an old binary of parsermodule.so saved away.