Expand test suite.

Expand the test suite.

Reorganized the example code into part of the test suite.

This test currently fails:

--- FAIL: TestReadFromPackedStream (0.00 seconds)
stream_test.go:38: {"year":0,"month":0,"day":0}
stream_test.go:38: {"year":0,"month":0,"day":0}
stream_test.go:31: ReadFromPackedStream: capn: too much data in stream

Looking into it.