Units: Use `use` to simplify implementing units

The unit implementations have a lot of method definitions that an implementation needs to define but that definitely just call out to a more common for code sharing.

For instance:

```elixir
def scale(count, unit) do
Scale.scale count, unit, __MODULE__
end
```

if we implemented that functionality inside a `__using__` definitions of those functions are unnecessary, leading to more concise unit definitions. A smaller example of this is implemented in #154

[genserver does the same](https://github.com/elixir-lang/elixir/blob/v1.3.0-rc.0/lib/elixir/lib/gen_server.ex#L437-L482) - well more or less. Difference is, we don't want the functions to be overridden as far as I can tell.

Downsides of this approach I see so far:

* magic :star2:
* I think the documenation of the methods that are defined "commonly" will be the same for all modules. Not really a "big" problem, but that means the iex usage examples/doctests will always contain examples of the other units (or none at all)

I think it should considerably shrink down the effort to implement new units so I think it'd be good. Interested to see what others think ( @wasnotrice since you did the first version here)

Not high priority, as it'd be pure refactoring