Make primitive implementations act on Const rather than Value
Remove unused import
Allow Execute to import primitives without cycle

The executor can now import Idris.Primitives without introducing an import
cycle. This involved moving elabPrims to the ElabDecls module.
Unify primitive implementations between evaluator and executor

The executor now uses the evaluator's implementations of primitives, with a
few minor special cases.
Special-case prim__believe_me

Because the unification of primitive ops required that primitives only work on
constants, believe_me needs to be special in both the evaluator and the
executor.
Merge pull request #358 from david-christiansen/unite-the-primitives

Unite the primitives