Generic or Kind-Agnostic linear system solvers

[FedericoPerini]

Absolutely @kimala, that’s exactly what I was trying to suggest here

@FedericoPerini

interface eigvals
        module procedure stdlib_linalg_eigvals_s
        ... ! other intrinsics interfaces
        module procedure stdlib_linalg_eigvals_la
end interface eigvals

The advantage of the interface block is compile-time path resolution, rather than the runtime resolution that you would have with a polymorphic version