Infer logp and logcdf of abs of discrete variables

[ricardoV94]

I was going to work on general icdf(abs(x)) which requires an optimization routine, but found some small crumbs along the way.

• Allow logp(abs(x)) and logcdf(abs(x)) for discrete x.
• Fix logcdf of DiscreteUniform for x == lower
• Handle special case in logdiffexp(-inf, -inf) so it can be safely used in more contexts