Best practice: How and where to put protocols and their implementation?

I think it would help to have a common place to put protocol definitions and also implementations. It would be in particular interesting if protocols are implemented for existing modules.

For discussion:

definition:

```
file: lib/protocols/incredible.ex
```

implementation for a new modules within the module.

implementation for an existing module (e.g. Integer) in:

```
file: lib/protocols/impl/integer.ex
```

or

```
file: lib/protocols/integer_impl.ex
```

?

Start by reading the discussion on this issue and compare the proposed paths, including lib/protocols/incredible.ex, lib/protocols/impl/integer.ex, and lib/protocols/integer_impl.ex. The work is complete only when the project agrees on a convention for protocol definitions and implementations and records it in the style guide.

Written by the indexing model from the issue text.