Simplify and test dispatch.jl 🐉 🐉 🐉

[odow]

The dispatch.jl file is only 47% tested: https://app.codecov.io/gh/jump-dev/MutableArithmetics.jl/blob/master/src/dispatch.jl

We should go through, remove the old hacks

MutableArithmetics.jl/src/dispatch.jl

Lines 751 to 777 in b46f175

	# Needed for Julia v1.1 only. If `parent(A)` is for instance `Diagonal`, the
	# `eltype` of `B` might be different form the `eltype` of `A`.
	function Matrix(A::LinearAlgebra.Symmetric{<:AbstractMutable})
	B = LinearAlgebra.copytri!(convert(Matrix, copy(A.data)), A.uplo)
	for i in 1:size(A, 1)
	# `B[i, i]` is used instead of `A[i, i]` on Julia v1.1 hence the need
	# to overwrite it for `AbstractMutable`.
	B[i, i] = LinearAlgebra.symmetric(
	A[i, i],
	LinearAlgebra.sym_uplo(A.uplo),
	)::LinearAlgebra.symmetric_type(eltype(A.data))
	end
	return B
	end
	function Matrix(A::LinearAlgebra.Hermitian{<:AbstractMutable})
	B = LinearAlgebra.copytri!(convert(Matrix, copy(A.data)), A.uplo, true)
	for i in 1:size(A, 1)
	# `B[i, i]` is used instead of `A[i, i]` on Julia v1.1 hence the need
	# to overwrite it for `AbstractMutable`.
	B[i, i] = LinearAlgebra.hermitian(
	A[i, i],
	LinearAlgebra.sym_uplo(A.uplo),
	)::LinearAlgebra.hermitian_type(eltype(A.data))
	end
	return B
	end

and add tests.

Once upon a time, I tried this #78.

My mistake was that I tried too much in one go and I got bogged down.