Add Ecto.Query.has_named_binding?/2

[AndrewDryga]

Related to #2389.

The main use case is preloading an association if it's not already preloaded:

  def with_preloaded_account(queryable) do
    if has_named_binding?(queryable, :account) do
      queryable
    else
      queryable
      |> join(:left, [assignment], account in assoc(assignment, :account), as: :account)
      |> preload([assignment, account: account], account: account)
    end
  end

We may also add a doctest with example usage:

  ## Examples

        iex> import Ecto.Query
        iex> query = from(p in "posts", as: :posts, \
                join: b in "blogs", \
                join: l in "links", as: :link)
        iex> has_named_binding?(query, :posts)
        true
        iex> has_named_binding?(query, :link)
        true
        iex> has_named_binding?(query, :blogs)
        false

[josevalim]

❤️ 💚 💙 💛 💜

[OvermindDL1]

Woooo! This was the last bit I needed to be able to get rid of all my state passing around and go back to base Ecto Query's and base Repo calls! Thanks much!