Add "equiv" tactic
"equiv t" asserts that the current goal has a type equivalent (by
conversion) to t
Allow 'rewrite ... in' to assert the resulting type
"rewrite f ==> t in x" means that after applying the rewrite f, the
required type of x must be t. This is primarily for readability of
proofs built by successive rewritings.
Merge branch 'master' of github.com:edwinb/Idris-dev