Binder: no public way to finalize or flush the interpreter from Kotlin #339

Found while writing the desktop release example (#338). Python3.finalize() is internal, so a consumer app cannot shut the interpreter down; Python's buffered sys.stdout is then lost at JVM exit unless the Python code flushes it itself (the example does). Also Python3.eval takes (expr, start, globals, locals) with no one-argument convenience for the common case. Completion: a public, documented shutdown (or an exit hook that flushes) and a simple eval form, with tests; README and the release example updated.