Annotations for extension API

[arnodirlam]

This changes the API for making existing modules (e.g. the standard library) compatible with Dx.

All functions that take functions as arguments must be implemented natively, i.e. not via defd but via explicit handling of Dx-internal return types, usually {:ok, ...} or {:not_loaded}.

To aid with the implementation, the Dx compiler provides an API to pre-process the arguments before they're passed to the functions, e.g. loading any scopes (used to lazily translate Elixir code to SQL). See the extension API docs for more info. The compiler can skip any unneeded functions, and perform other optimizations.

This API is currently defined via a callback function, __fun_info(fun_name, arity).

This PR introduces annotations that are co-located with the function definitions, instead of defining all callback clauses at the top of the module, and implementing the functions further below. See the changed in Dx.Enum in this PR for an example.

To pick up the annotations, def must be changed to defd_, standing for "native defd". The only difference currently being that it supports the annotations.

Further options

These options are non-exclusive:

Allow annotations for def (removing defd_)

This would make it more intuitive, as it's in line with libraries such as decorator.

On the other hand, keeping defd_ could be used to define modules with a mix of defd, defd_ and def (not compatible with defd) functions.

Rename the callback & annotations to __dx_compile__ and @dx_compile

This would make it clearer that it's an interface for instructions to the Dx compiler.

[arnodirlam]

Now going all-in with the _ suffix naming. Better to have something consistent for now than anything else.

defmodule Dx.Enum do
  use Dx.Defd_

  @dx_ args: [[:atom_to_scope, :preload_scope], :final_args_fn]
  defd_ map(enum, fun) do
    # ...
  end
end