implementation of ctx_substD.
rename subst functions to have subst_ prefix

- this makes them easier to look for.
removing dead code.
SubstInstatiatable.
proofs in rtac core.
hit a snag thinking again.
checkpoint.
working on contexts (currently broken)
more revisions.
stronger well-formedness predicate for ctx_subst
checkpoint.

- need to generalize instantiate to any [Pure-able] thing.
massive progress through rtac.
some more cleanup.
proofs of assumption (actually eassumption)
cleanup the proofs in Intro

- variables still have a few loose ends.
Decidable equality for RType

- it is implied by the interface anyways.
Reduce proofs.
starting to look at eapply.
unifying mentions into mentionsAny
the first fully-building Rtac in a while.