simplify isSet<SomeAlgebraicStructure>

[MatthiasHu]

This PR simplifies, or at least harmonizes, the definitions of the following lemmas:

isSetGroup
isSetRing
isSetDistLattice
isSetCommAlgebra
isSetLeftModule
isSetField
isSetAlgebra
isSetLattice
isSetCommRing
isSetAbGroup
isSetOrderedCommMonoid
isSetCommMonoid

Each of them is a simple is-set after opening the respective <Something>Str. In the case of Algebra and OrderedCommMonoid, the is-set field was impossible to use since it was exported twice from the respective records; I fixed that by hiding one copy (as it was already done in Ring, for example).

[felixwellen]

Thanks for the nice refactoring!