remove req_value

[Negabinary]

I noticed we don't really use req_value any more in the evaluator, because we mostly rely on unboxing which gives us finer-grained control.

e.g. [?] is not a value, but that doesn't mean we can't do list operations on it, so evaluation shouldn't be waiting for sub-expressions to be values, they should only check whether they can unbox the desired kind of value.

ValueChecker is staying because we still use it in $v stepper filter patterns.