Use heterogeneous equality for take/drop proofs
Use heterogeneous equality for take/drop proofs

The old versions with type level rewrites were incredibly messy and
would likely get even uglier at some use sites. The new ones are much
nicer.

kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev