RFC: Deprecate `util.array.searchsorted`

[oyamad]

Deprecate searchsorted(a, v) in util/array.py and replace it with np.searchsorted(a, v, side='right').
(Numba was not supporting the side keyword argument when this function was implemented.)

• Replace any use of util.array.searchsorted with np.searchsorted(a, v, side='right').
  (⚡️ Speed up function draw by 390% #811 Is such an example.)
• Rewrite searchsorted in util/array.py so that it simply returns np.searchsorted(a, v, side='right'), emitting DeprecationWarning.
  (Remove this function in some future release.)
• Rewrite the docstring of searchsorted in util/array.py accordingly.