Add basic functionality for univariate Ore operators

[ooinaruhugh]

This pull request adds basic functionality for univariate Ore polynomials. This is implemented in pure Julia.
For now, this contains arithmetic capabilities and implements an interface similar or equal to NCPolyRing and such.

There are some minor additions that are technically required that would come with this PR.
Currently, there is no way to define a ring homomorphism of fraction fields (or function fields for that instance).
They are a rather common choice for coefficient fields of Ore algebras, that's why they are of interest.
Since for fraction fields of polynomial rings, the notion would naturally extend, I also introduce a type PolyFracFieldAnyMap that mimics PolyRingAnyMap.

I would keep this PR at this scope since this is also the point where we fix the interface for Ore algebras
which will be relevant as soon we make use of the implementation in FLINT for some specialised cases.

[ooinaruhugh]

As long as this is a draft, I am ignoring some style rules. I didn't sort the exports.jl so I don't lose track of what I've added

[lgoettgens]

Suggested change

	function PolyFracFieldAnyMap{D,C,V}(d::D,ϕ::V) where {D,C,V}
	return new{D,C,V}(d,ϕ)
	end
	function PolyFracFieldAnyMap{D,C,V}(d::D,phi::V) where {D,C,V}
	return new{D,C,V}(d,phi)
	end

please do not use unicode in variable names, see https://docs.oscar-system.org/dev/DeveloperDocumentation/styleguide/#Unicode