Operator overloading for Unit.Quantity types.

use Unit.Operators in a module to get unit-aware +, -, <, >, <=, and >=. Operands that are not unit quantities fall straight through to Kernel, so the rest of the module's integer arithmetic and comparisons are untouched. Equality (==) is left as Elixir's native struct equality, which already does the right thing for these single-field structs.

defmodule Scheduler do
  use Unit.Operators

  def headroom(total, used), do: total - used         # Information - Information
  def fits?(avail, need), do: need <= avail           # Information <= Information
  def retries(n), do: n + 1                           # plain integers, via Kernel
end

Combining two different dimensions (Information + Time), or a quantity with a bare number, raises ArgumentError -- the whole point of the unit types is that those mistakes do not silently compute.