[mod] [BREAKING] Move to more flexible packer interface (wip)

This is a BREAKING change for anyone using a CUSTOM packer
(i.e. anything besides the built-in :edn or transit packers).

The OLD interface (IPacker):
  (pack   [packer    clj-val]) -> platform-binary/string
  (unpack [packer packed-val]) -> clj-val

The NEW interface (IPacker2):
  (pack   [packer websocket?    clj-val cb-fn]) -> (cb-fn {:value <platform-binary/string>} or {:error <platform-error>})
  (unpack [packer websocket? packed-val cb-fn]) -> (cb-fn {:value <clj-val>                 or {:error <platform-error>})

Motivation? The new interface:

  - Allows custom implementations depending on transit type (WebSocket/Ajax).
  - Supports binary encoding for both WebSocket and Ajax transit
    (previously only WebSockets supported binary encoding).
  - Asynchronous encoding, opening a lot more options for client-side
    serialization and/or compression. Many JS serialization and compression
    libs are async only.

Need to migrate your custom IPacker->IPacker2?

  Migration should usually be straight-forward, but if you run into any
  unexpected issues please feel free to ping on the Sente Slack channel or
  on GitHub. Both are linked at https://www.taoensso.com/sente.

  - Peter Taoussanis

Author: ptaoussanis