Revert "Disable constants sharing in eq engine, disable hack in theory engine."

This partially reverts commit f70804a7159390fcb01d8c1ec208fbfd8e544fba.

In particular, it DOES NOT revert
  "Changes to strings solver : modify lemmas/splits to avoid constants, minor refactoring.  Fix assertion failure in quantifiers engine."
which is part of the same commit.
Merge branch 'master' of https://github.com/CVC4/CVC4