Move src/Infinity.jl (and tests) from Nemo to AbstractAlgebra

[fingolfin]

No description provided.

[dlfivefifty]

While mathematically an infinity isn't a real number

As you mentioned: Julia types are not the same as mathematical sets. I think they are best thought of as encoding an "interface" and anything that follows the rules of the interface can be subtype. Eg. in ForwardDiff.jl we also have Dual <: Real which of course is mathematically nonsense but makes perfect sense (and is essential) in terms of the interface.

I don't exactly remember why we defined Infinity <: Real instead of Infinity <: Number or indeed Infinity <: Any. This can probably be changed without any serious side-effects.

[fingolfin]

No we have no interest in that. It just makes maintenance harder for us with no real benefit. As you pointed out, there is already a package for infinites that people who don't want to use AA/Nemo/Oscar can use, and which deliberately are not using.

In particular our infinity is not a subtype of Real. That's not a bug, it's a feature.

"Consistent hashing" of infinities to me also sounds like a red herring, not a useful feature in itself.