Inexact conversion for rational powers

I just tried
```julia
uconvert(u"m^-1", 1"pl^(-1//3)")
# returns 99999.99999999999 m^-1
uconvert(u"μm^-1",1.0u"1/pl^(1//3)")
# returns 0.09999999999999998 μm^-1
```
and I get these floating point round-off errors.
Would it be possible to get exact values, if we'd define special handling of `cbrt` like there is for `sqrt`?

Start by tracing the uconvert handling for rational powers and compare its treatment of cbrt with the existing special handling for sqrt. Confirm the behavior with the two Julia examples in the issue; done means the conversions return exact values rather than floating-point round-off results.

Written by the indexing model from the issue text.