Improve qrt::exp() kernel composition time

Mccaskey, Alexrequested to merge
github/fork/tnguyen-ornl/tnguyen/exp-qrt into master

Created by: tnguyen-ornl

Fixes https://github.com/ORNL-QCI/qcor/issues/92

Also, add a unit test to make sure QCOR's exp kernel matches the reference XACC circuit.

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