Fix pfrexp/pldexp for half. The recent addition of vectorized pow (!330) relies on `pfrexp` and `pldexp`. This was missing for `Eigen::half` and `Eigen::bfloat16`. Adding tests for these packet ops also exposed an issue with handling negative values in `pfrexp`, returning an incorrect exponent. Added the missing implementations, corrected the exponent in `pfrexp1`, and added `packetmath` tests.
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