[mod] [BREAKING] Switch 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-exception})
  (unpack [packer websocket? packed-val cb-fn]) -> (cb-fn {:value clj-val                 or {:error platform-exception})

Motivation? The new interface supports:

  - Distinct un/packing for WebSocket and Ajax transit.
  - Proper support for binary packing, even over Ajax.
  - Async un/packing, opening a lot more options for client-side
    serialization and/or compression.

Author: ptaoussanis