Add #one to HasMany (#550)

[landongrindheim]

Per #550, the HasMany interface
does not line up with Hanami's guides. Prior to this commit, the
following is invalid as HasMany does not implement #one.

class AuthorRepository < Hanami::Repository
  associations do
    has_many :books
  end

  def find_book(author, id)
    book_for(author, id).one
  end

  private

  def book_for(author, id)
    assoc(:books, author).where(id: id)
  end
end

This implementation is based on a couple of others, notably ROM's and
Ecto's (Elixir). I opted to name the raised error MultipleResultsError,
which is the exact name used by Ecto, because I feel it bears a semantic
relationship to the name of the method from which raises it.

Note: This is one of two solutions to the linked issue, the other being #554. If either is accepted, the other PR should be closed.

Closes #550

[landongrindheim]

@jodosha Could you let me know if this (or #554) is something you're interested in? If not, I'll close the PRs to clear them from my GitHub dashboard.

[landongrindheim]

Closing this pull request to clear it from my GitHub dashboard.

@jodosha feel free to re-open this if you're interested in using this to address issue #550