Cholesky accessors of adjoints should return views

[simonbyrne]

Currently accessing the lower triangular portion of a Cholesky object which has been stored using its upper triangle (or vice-versa) will return a copy

julia/stdlib/LinearAlgebra/src/cholesky.jl

Lines 342 to 350 in c887c74

	if d == :U
	return UpperTriangular(Cuplo === char_uplo(d) ? Cfactors : copy(Cfactors'))
	elseif d == :L
	return LowerTriangular(Cuplo === char_uplo(d) ? Cfactors : copy(Cfactors'))
	elseif d == :UL
	return (Cuplo === 'U' ? UpperTriangular(Cfactors) : LowerTriangular(Cfactors))
	else
	return getfield(C, d)
	end

To avoid unnecessary allocations, you end up with code that looks like this:
https://github.com/JuliaStats/PDMats.jl/blob/eeec1c1bfdddb8f522952f81b3f6a3d5ec8aa7bd/src/pdmat.jl#L57-L59

The logical solution is that it should return an Adjoint object. However this would no longer be type-stable, since the flag is stored as a value part of the Cholesky struct.

Possible solutions:
a) include the flag as part of the type parameter, either directly as a symbol, or store the factor field as a LowerTriangular/UpperTriangular.
b) get rid of the option and only ever store it as an UpperTriangular matrix.