Unwrap 'as' constructs

[Chris Warburton]

The TIP format allows type annotations, presumably to help tools which
struggle with inference. It's used like like:

  (as nil (list int))

This is the value 'nil', but annotated with the type '(list
int)'. Presumably this is useful because 'nil' is polymorphic: it works
for any element type, whereas this expression might appear in a location
which requires a list with element type 'int'.

In any case, we do not expect our theory exploration tools to sprinkle
type annotations around their conjectures, so we should strip these out
before comparing.