Make our custom matrix types GPUArrays-compliant

[gdalle]

As shown in #39, the new sparse interface in GPUArrays can be useful, we should try to implement it (and eventually upstream it).

Related:

• Conversion to native sparse formats albertomercurio/GenericSparseArrays.jl#8
• Assumptions in abstract sparse array interface JuliaGPU/GPUArrays.jl#648
• Add direct constructors for sparse arrays JuliaGPU/GPUArrays.jl#684