Fix missing copyto! methods for triangular matrix types

[jishnub]

The _copyto! methods were not defined for all combinations, and copyto! was also too permissive (it accepted mismatched uplos). This PR adds the missing methods, and updates the specialized copyto! such that it only accepts combinations with matching uplos. The other cases will be handled by the fallback method.

Fixes, e.g.:

julia> using LinearAlgebra

julia> A = [1 2; 3 4];

julia> copyto!(UpperTriangular(A), UnitUpperTriangular(A))
ERROR: MethodError: no method matching _copyto!(::UpperTriangular{Int64, Matrix{Int64}}, ::UnitUpperTriangular{Int64, Matrix{Int64}})
The function `_copyto!` exists, but no method is defined for this combination of argument types.
[...]

After this,

julia> copyto!(UpperTriangular(A), UnitUpperTriangular(A))
2×2 UpperTriangular{Int64, Matrix{Int64}}:
 1  2
 ⋅  1