Theory and instances for FdVect with both of its monoidal products

Just recording this from a comment from @epatters on Zulip:

> [snip] Note that [graphical linear algebra](https://github.com/AlgebraicJulia/Catlab.jl/blob/master/src/linear_algebra/GLA.jl) is about `(FdVect,⊕,0)`, not `(FdVect,⊗,k)`, i.e., the monoidal product is the direct sum, not the tensor product.
>
> Like you said, it'd be good to eventually have a clean, mathematically respectable implementation of FdVect with both of its monoidal products. Currently things are a bit scattered. Another starting point is [here](https://github.com/AlgebraicJulia/Catlab.jl/blob/565f7d56ea7b14bb546ca3fc154694c0c64317c6/src/categorical_algebra/Matrices.jl#L19), implementing both monoidal products for standard Julia matrices over a commutative rig.