This change adds the following:
- syntax `deriving instance ...` for automatic derivation of
  instances/dictionaries
- constructor `NewtypeDict` for dictionary expressions
- synthesis of `NewtypeDict` dictionaries
- (currently incorrect) simplification of method applications on
  `NewtypeDict dictionaries

Methods are constructed by first synthesizing a dictionary for
the instance that is derived from. Those methods are then converted
to methods suitable for the derived instance by using an isomorphism
between types.

This constructor was introduced for automatic derivation of
instances. This constructor is no longer needed.

This constructor was introduced for automatic derivation of
instances. This constructor is no longer needed.

Specifically, by representing isomorphisms as abstractions (`Abs`) we
avoid the need for evidence of `Emits`.

Also fix the type of an `App` expression that is constructed
during method conversion.

This simplification has led to the removal of an unsafe coercion.

…tion.

The current implementation of automatic instance derivation synthesizes
methods (for the derived instance) in a way that is guided by the method
types in the instance that we are deriving _from_. For the method synthesis
to work correctly certain types (i.e. the type we are deriving _from_ and
the type we are deriving an instance _for_) must not be mentioned in the
type of the class method that corresponds to the instance method that is
being synthesized.