Minor test cleanup
streams/changed: Fix arbitrary fns.

The arity detection patch only worked for keywords, not arbitrary
functions.
streams/changed: Emit pairs instead, with the :pairs? option.

Realized that the dual-arity function made streams/changed difficult to
compose with other primitives like smap; backing off to emitting
vectors.
Require riemann.streams, folds, test/io, test/tap in tests.
Moved test funcs from stream_test.clj to test.clj
run-stream now advances time.