Error flag for case expressions' associated DHExp

[pigumar1]

This pull request aims to merge the three different case constructs in DHExp into one case construct.

The new approach will replace
DHExp.ConsistentCase(DHExp.case),
DHExp.InconsistentBranches(MetaVar.t, HoleInstanceId.t, DHExp.case),
DHExp.InexhaustiveCase(MetaVar.t, HoleInstanceId.t, DHExp.case) (added for #1124),
and DHExp.case.Case(t, list(rule), int)

with DHExp.Case(t, list(rule), int, option((MetaVar.t, HoleInstanceId.t))), in which the last parameter indicates whether the corresponding case expression is inconsistent.

Two concerns:

• Do we need to preserve the ability to distinguish consistent/inconsistent case expressions with DHExp.constructor_string? If so, the new Case construct, as well as the function DHExp.constructor_string would be a little bit more complex.
• Do we need to preserve the ability to have different inconsistent case expressions behave (dynamically) differently? If so, option might not be enough.