move the reduction control of the proofmode to its own file and tweak it
* move PROP-envs definitions to environments.v so that we can control them without pulling in coq_tactics * use reduction-controlled `pm_default` for proofmode accessors
* move PROP-envs definitions to environments.v so that we can control them without pulling in coq_tactics * use reduction-controlled `pm_default` for proofmode accessors