Change syntax for newtypes in External Core

Change syntax for newtypes in External Core

The way that newtype declarations were printed in External Core files was
incomplete, since there was no declaration for the coercion introduced by a
newtype.

For example, the Haskell source:

newtype T a = MkT (a -> a)

foo (MkT x) = x

got printed out in External Core as (roughly):

  %newtype T a = a -> a;

  foo :: %forall t . T t -> t -> t =
    %cast (\ @ t -> a1 @ t)
    (%forall t . T t -> ZCCoT t);

There is no declaration anywhere in the External Core program for :CoT, so
that's bad.

I changed the newtype decl syntax so as to include the declaration for the
coercion axiom introduced by the newtype. Now, it looks like:

  %newtype T a ^ (ZCCoT :: ((T a) :=: (a -> a))) = a -> a;

And an external typechecker could conceivably typecheck code that uses this.

The major changes are to MkExternalCore and PprExternalCore (as well as
ExternalCore, to change the External Core AST.) I also corrected some typos in
comments in other files.

Documentation and external tool changes to follow.