internal/core/adt: handle incomplete values in comparators

Previously, incomplete values were not correctly
identified for recursive values like list elements.

We fix this by validating incoming errors
recursively using Validate.

As this eliminates incomplete values from the
input, this also allows us to use Equal to compare
lists.

Some of the errors that should be incomplete, now
are, which is good.
Some of the errors that should be final are still
incomplete, this is okay.
In general it is okay to be a weaker error when
a stronger error is allowed.

Note that this changes the semantics of newBool,
which is fine, as it only is used by binOp.

Issue #2583

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I596b927fbbb801a3dd0d8f331a211c2b081afca8
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1217008
Reviewed-by: Roger Peppe <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mpvl