Code style issues in the foreign API

A pass on the API code revealed the following style issues:

* chained calls (Stream/Optional) not always occur on separate lines
* parameter names sometimes are inconsistent
* inconsistent cast formatting (e.g. (Foo)foo or (Foo) foo)
* Use static imports for Objects.requireNonNull() [optional - JDK is inconsistent about this]
* inconsistent class naming in internal packages
* methods and classes should be final where possible
* Methods and properties should be sorted in some order
* interfaces should be used where possible (this should already be the case, especially after latest changes to layout API)
* Cleanup the way method parameters (e.g. one on each line) and class declarations (e.g. extends, implements, permits)
* Generally make classes smaller (move to util methods for example)
* Move out some default implementations from interfaces to internal util methods