erf: remove redundant allocations in Dense MLE concat, fix_variables,and evaluate

[Fibonacci747]

[concat] Use from_evaluations_vec to move the built Vec instead of cloning via from_evaluations_slice, eliminating one full-buffer copy.
[fix_variables] After folding, truncate to 1 << (nv - dim) and return with from_evaluations_vec to avoid cloning the sliced prefix.
[evaluate] Compute the scalar directly by folding over a temporary buffer (cloned once from evaluations) and return a[0], avoiding constructing a new DenseMultilinearExtension

[weikengchen]

You make a good point: the a few copying is actually a bit bad. Will review!