quick sketch of composable continuation implementation

I've been told by knowledgeable people that it is impossible to
implement composable continuations (AKA delimited continuations AKA
shift/reset) in terms of call-with-current-continuation.  Since I
don't yet understand why that is, I figured it would help my
understanding to attempt it and see how it fails.
simplify shift/reset API and add test (currently failing)
fix handling of multiple shifts delimited by a single reset
revert unintended change to jnienv.cpp
fix openjdk and android builds when continuations=true

Also, update the whitespace padding for printing test results to
accommodate long names like extra.ComposableContinuations.
remove redundant interfaces and generalize shift/reset generics

Turns out Function can do the jobs of both CallbackReceiver and
FunctionReceiver, so I've removed the latter two.

Also, shift and reset should work with a combination of types, not
just a single type, so I've expanded their generic signatures.
return naturally from function in Continuations.shift rather than via the continuation

Since the function in question is the only one on the call stack above
the reset method, there's no need to invoke the captured continuation
-- we get the same effect by just returning normally, and it's more
efficient that way.
remove makefile references to deleted classes
consolidate duplicate Cell classes
Merge pull request #207 from dicej/composable-continuations

Composable continuations