varying/test+bug #4: add first tests for flatMap, fix related bugs.
varying/test #4: more (all?) tests for flatMap.
varying/test+bug #4: test remaining extant code; fix up ::pure().
varying/new #4: implemented ComposedVarying. not tested, but didn't break anything.
varying/test+bug #4: tests and fixes for Varying::pure.
varying/test #4: start of tests for Varying::flatMapAll.

* added tests for a new known bug w/ flattened reactNow, but currently
  skipped as the fix hasn't been worked out yet.
varying/test+bug #4: fix up flatMap reactNow extant-value bug.