Move VariablesExpOpt to dslapi.

- Now, this actually doesn't affect SQL queries, but regexp matchers.
- Seems OK, basically optimized some dead code (initial val for unused reads).
- Gain is actually only one line of code per generated matcher code file.

BEFORE:
wc sre__bab_s.check.scala sre__bab_s_e.check.scala sre_a_sb.check.scala
sre_hel_s.check.scala sre_hel_s_e.check.scala
      62     195    1295 sre__bab_s.check.scala
      64     207    1348 sre__bab_s_e.check.scala
      84     280    1790 sre_a_sb.check.scala
      94     308    2114 sre_hel_s.check.scala
      96     320    2174 sre_hel_s_e.check.scala
     400    1310    8721 total

AFTER:
wc sre__bab_s.check.scala sre__bab_s_e.check.scala sre_a_sb.check.scala
sre_hel_s.check.scala sre_hel_s_e.check.scala
      61     191    1275 sre__bab_s.check.scala
      63     203    1325 sre__bab_s_e.check.scala
      83     276    1770 sre_a_sb.check.scala
      93     304    2090 sre_hel_s.check.scala
      95     316    2150 sre_hel_s_e.check.scala
     395    1290    8610 total