Improve generics (covariance and contravariance)
Expand TCK to combinatory flows and fix issues
Improve javadoc and group Stream modifiers (returning this)
Refactor TupleN to fit more naturally within unlimited arrays and operations such as Stream.zip
Fully type Streams.zip and Streams.join up to 8 generics T1..T8
Misc minor fixes
Separate Callback observe and terminal consume behaviors
Fix CombineAction and Action.combine() to handle FanInAction
Misc fixes to get TCK validate a more complex flow