Pass mutable copies to inplace LinearAlgebra functions

[jishnub]

This gets some functions working with immutable matrix types, e.g.:

julia> using FillArrays, LinearAlgebra

julia> F = Fill(big(2), 4, 4)
4×4 Fill{BigInt}, with entries equal to 2

julia> det(F)
0

julia> triu(F)
4×4 Matrix{BigInt}:
 2  2  2  2
 0  2  2  2
 0  0  2  2
 0  0  0  2