auth-source.el rewrite

auth-source.el rewrite

* auth-source.el: Bring in assoc and eioeio libraries.
(secrets-enabled): New variable to track the status of the Secrets API.
(auth-source-backend): New EIOEIO class to represent a backend.
(auth-source-creation-defaults): New variable to set prompt defaults
during token creation (see the `auth-source-search' docstring for
details).
(auth-sources): Simplify to allow a simple string as a netrc backend
spec.
(auth-source-backend-parse): Parse a backend from an `auth-sources' spec.
(auth-source-backend-parse-parameters): Fill in the backend parameters.
(auth-source-search): Main auth-source API entry point.
(auth-source-delete): Wrapper around `auth-source-search' for deletion.
(auth-source-search-collection): Helper function for searching.
(auth-source-netrc-parse, auth-source-netrc-normalize)
(auth-source-netrc-search, auth-source-netrc-create): Netrc backend.
Supports search, create, and delete.
(auth-source-secrets-search, auth-source-secrets-create): Secrets API
backend stubs.
(auth-source-user-or-password): Call `auth-source-search' but it's not
ready yet.