Proposal: CombinedTrait

[matbesancon]

This could be a simpler alternative to #2

Defining a trait CombinedTrait{X, T1, T2} with T1{X}, T2{X} traits.

The implementation of CombinedTrait{X, T1, T2} could be given by a function combine_trait implemented as trait_func1(X) && trait_func2(X) (I am not sure if this is feasible)

On the user side, a combined trait can be specified on a function with:

@traitfn function hello(g::G) where {G <: Thing; CombinedTrait{T1{G}, T2{G}}}