internal/core/adt: comparison of values of different types

This was an error before, but now returns false
if `@experiment(structcmp)` is enabled.

Note that only the tests in 016_comparison.txtar
are affected, as it specifies the experiment.
There are many other tests that verify that
comparison to null still works if this
experiment is not enabled.

Issue #2583

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I5dd1cffebbe6fde1f28cf2fb3725d1ed8e6ea0a7
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1217010
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl