One multiplication too many in `_exp!`

[jarlebring]

Analogous to the PR: JuliaLang/julia#40668 I believe the _exp!-function can be implemented with one multiplication less in several cases. More precisely, these lines can be implemented with one multiplication less:

ExponentialUtilities.jl/src/exp.jl

Lines 41 to 49 in 5460b95

	mul!(A2, A, A)
	@. U = C[2] * P
	@. V = C[1] * P
	for k in 1:(div(size(C, 1), 2) - 1)
	k2 = 2 * k
	mul!(temp, P, A2); P, temp = temp, P # equivalent to P *= A2
	@. U += C[k2 + 2] * P
	@. V += C[k2 + 1] * P
	end

Since when k=1, P is identity.