Experiment with Transit PR (1139ec0) integration

Goals:
  * Optional Transit dependencies.
  * Better performance for small data. In practice it seems
    this means we're going to want to allow Sente users to
    mark payloads as big/small so that the optimal packaging
    method can be selected. Sente can continue to use edn
    for its own internal (+small) payloads.
Add extension-point interfaces, make Transit stuff optional
Reference example: misc housekeeping
Packing: allow payload meta to transfer to package meta
Optimization: use mini uuids for callbacks
Optimization: minify event messages + simplify event wrapping
v0.16.0-SNAPSHOT