Fix unnecessary warning emitted from muchused.ml

Problem

There was a missing checks for chest & chest_key in muchused.ml related to is_dup,

That was emitting a warning which chest or chest_key was used more than once.

Solution

The simple solution would be to add the 2 missing checks,

But in order to prevent such error in future the constant types are defined as a Variant,

Such errors will be easily caught by exhaustiveness check of pattern matching

• has a changelog entry