Allow users to export theorem names that are constructor names.

The hacky trick to allow this is to first use a fun binding to
"de-constructorify" the name. Thus

fun Empty x = x
val Empty = 3

allows you to bind Empty, despite it being an exception name, and thus
a constructor name.

There are still names that just won't work: Poly/ML (at least) doesn't
allow rebinding of ::, true, false, ref and nil. We should probably
immediately raise an error with those cases on an attempt to save.

Closes #225