Tests & Refactor

• add tests
• implement build in terms of sync-derivatives
• add assertions that spec is a map
• when checking if something is a function to use to derive a new
  value or if it is a source we now use (implements? IWatchable x)
  instead of (fn? x)
• refactor pooling implementation to allow testing of internals
• Rum mixins: simplifiy keys used in childContext to be strings
• Rum mixins: make sure the token is correctly passed to get! and release! functions