Inject matched variables into `where` predicate scope

Predicates written in `@term RULES` using the `where` clause could benefit from some syntactic sugar. Rather than having to explicitly index into the given match, the scope could automatically be aware of the variables.

```julia
(abs(x) => x) where {σ -> isvalid(Image(σ[x], Nonnegative))}  # current
(abs(x) => x) where {isvalid(Image(x, Nonnegative))}  # proposed
```