Unit type computations

It would be nice to be able to compute on the type directly:

```julia
using Unitful
uType = typeof(1u"kg")
tType = typeof(1u"s")
uType/tType
```

This would allow the computation of where types as well:

```julia
using Unitful
uBottomEltype = Unitful.Quantity{Float64,D,U} where U where D
tType = typeof(1u"s")
uBottomEltype/tType
```

Currently, handling recursive arrays is incompatible with Unitful due to the inability to do this calculation.

Start by reproducing the two Julia examples using Unitful.Quantity types and inspect how the library currently handles unit arithmetic on values versus types. Done means the shown type-level divisions compute correctly, including the parameterized uBottomEltype case, and recursive arrays are compatible with Unitful.

Written by the indexing model from the issue text.