pure and ipure for Form are inconsistent

For `Form`, `pure` function returns a `Form` with `Proved () (FormRange i i) a`, while `ipure` function returns a `Form` with `Proved p (unitRange i) a`. `FormRange i i` is a zero-width range, and it's not equal to `unitRange i`.

I think `pure` function is wrong because the `FormRange` in the `FormState` would never be a zero-width range in normal operation.