Decicated version of string decider for the proof mode.

This way we avoid the env_cbv tactic unfolding string related stuff
that appears in the goal and hypotheses of the proof mode.
1 job for master in 9 minutes and 51 seconds (queued for 7 minutes and 58 seconds)
Status Job ID Name Coverage
  Test
passed #1302
coq
buildjob

00:09:51