Improve erc-button--modify-nick-function interface

Improve erc-button--modify-nick-function interface

* lisp/erc/erc-button.el (erc-button--check-nicknames-entry): Remove
unused let binding.
(erc-button--preserve-bounds): Remove unused function.
(erc-button--nick): New struct type to serve as collection plate for
`erc-button--modify-nick-function' consumers.
(erc-button--modify-nick-function): Reexplain interface, now based on
`erc-button--nick' object.  Change default value to `identity'.
(erc-button--add-phantom-speaker): Redo to expect `erc-button--nick'
object.
(erc-button-add-nickname-buttons): Rework slightly to construct an
`erc-button--nick' object for feeding to
`erc-button--modify-nick-function'.  Only run the latter when an
`erc-server-user' has successfully been found.  (Bug#60933)