distinct types for positive and negative (Real) infinity

In order to make use of this package for TransformVariables.jl, I would like to ask for distinct types for *positive* and *negative* (real) infinity, ideally accessible with `∞` and `-∞`. I need types I can dispatch on for type stability.

Basically [this code](https://github.com/tpapp/TransformVariables.jl/blob/260ea7ecf987435b3289d5d68fb90202182ff262/src/scalar.jl#L137-L149) does what I need.

(cf [discussion](https://discourse.julialang.org/t/name-collision/55033/8?u=tamas_papp))