The 'is' function doesn't work on terms

[Matt Giuca]

Currently is(x, y) where x and y are terms (values of a user-defined type) just returns 0. This is because interpret.ref_eq_vals has no way of checking the identity of a term. For implementing field set (bug #439171), terms will need to have an extra layer of indirection anyway, so as part of this, fix ref_eq_vals to perform an identity check on the terms.

Also update documentation for is (in impure module), removing the note about this bug.

[Matt Giuca (mgiuca)]

Fixed in trunk r1090.