Quantities should be <: Real?

Currently, Unitful quantities are <: Number but not <: Real. This hinders composability with other libraries.

Just a few examples for methods that accept Reals and don't work with units - but actually should and totally make sense:
- `Base.searchsorted(::AbstractRange{<:Real}, ::Real)` does division instead of performing binary search - so, it's faster than AbstractArray fallback
- `StatsBase.weights` require `AbstractVector{<:Real}`

Surely there are much more.
Note that these methods are correctly restricted to reals: they don't make sense for complex numbers.

Why not make Quantity <: Real, and use Complex{Quantity} for complex unitful values?