reduce size of struct regmatch_state

reduce size of struct regmatch_state

Currently the trie struct is the largest sub-struct in the union;
reduce its size by 2 x 8 bytes (on a 64-bit system) by
    1) aligning a bool better
    2) eliminating ST.B, which can be trivially derived as
       ST.me + NEXT_OFF(ST.me)

(I'm going to partially spoil this by adding a new field in the next commit)