Give MathCell arithmetic ops implementations

[bluss]

This is a follow-up to MathCell introduced in #877.

MathCell should have arithmetic operations so that arrays and views using MathCell elements can participate in some arithmetic ops. Preferably together with non-cell arrays. Detailed design might need to be decided after seeing what's permitted w.r.t impl overlap.

T.b.h most-wished for items would be ArrayView<MathCell<T>, _> += &array<T, _> and similar - i.e. easy ways to modify ArrayView<MathCell<T>, _> using assign ops.