internal/core/adt: use a named vertexStatus constant rather than 0
"unprocessed" is much clearer than 0, and since it's not an untyped
integer constant, it cannot be mixed up for other types.
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I1664686a59c8a69e9e3c493bc785de072babaf89
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198625
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Matthew Sackman <matthew@cue.works>