ensure `FixedSizeArray` is performant with LinearSolve.jl

[nsajko]

TODO for once FixedSizeArrays.jl gets registered: ensure performance with LinearSolve.jl.

I suppose LinearSolve would ideally treat FixedSizeArray like it treats Array. An example where this is not so:

julia> using LinearSolve, FixedSizeArrays

julia> let A = FixedSizeArrayDefault{Float32}
           LinearSolve.defaultalg(A(undef, 3, 3), A(undef, 3))
       end
LinearSolve.DefaultLinearSolver(LinearSolve.DefaultAlgorithmChoice.KrylovJL_GMRES)

julia> let A = Array{Float32}
           LinearSolve.defaultalg(A(undef, 3, 3), A(undef, 3))
       end
LinearSolve.DefaultLinearSolver(LinearSolve.DefaultAlgorithmChoice.GenericLUFactorization)