add literal_pow overload

[aplavin]

Mostly for performance:

f(x) = x^2

x = Particles()
xu = x*u"km"

@b f(x)  # goes from a few mus to hundreds of ns
@b f(xu)  # goes from a few ms (!) to hundreds of ns

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.05%. Comparing base (6ca57ed) to head (37edce5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
- Coverage   90.28%   90.05%   -0.24%     
==========================================
  Files          20       20              
  Lines        1153     1156       +3     
==========================================
  Hits         1041     1041              
- Misses        112      115       +3     

Flag	Coverage Δ
unittests	90.05% <100.00%> (-0.24%)	⬇️

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[baggepinnen]

Thanks for the PR! I merged your commits with an additional test in #151