Extract unit to apply to new value

Could there be a simpler way to obtain a constructor of the same unit as another value.
For example,
```
get_unit(3u"mg/hr")(3.5) == 3.5u"mg/hr"
```
Something like,
```
using Unitful, Distributions

apply_unit(obj::Real, val::Real) = val
apply_unit(obj::Quantity{<:Any,D,U}, val::T) where {T,D,U} = Quantity{T,D,U}(val)
magic(obj) = apply_unit(obj, mean(rand(Normal(ustrip(obj)), 100)))
```

No file or test is named. Start by inspecting the existing Quantity and unit-extraction APIs, along with the proposed get_unit and apply_unit entry points; done would be a supported way to construct a new value with the same unit as an existing value, matching the example.

Written by the indexing model from the issue text.