This BIG PATCH contains most of the work for the New Coercion Representation

See the paper "Practical aspects of evidence based compilation in System FC"

* Coercion becomes a data type, distinct from Type

* Coercions become value-level things, rather than type-level things,
  (although the value is zero bits wide, like the State token)
  A consequence is that a coerion abstraction increases the arity by 1
  (just like a dictionary abstraction)

* There is a new constructor in CoreExpr, namely Coercion, to inject
  coercions into terms