Added fibrous.js with tests.

From the docs for fibrous:

// Return a wrapped 'fibrous' version of the given function that takes
// a callback as a MANDATORY final argument, and doesn't take a function
// as any of its other arguments.
//
// If the fibrous version is run in a Fiber and not given a callback,
// it will call its wrapped function with a Future, wait until the
// Future is resolved, and return the result (or throw an exception).
//
// If the fibrous version is not called in a Fiber, or is passed
// a callback, the wrapped function will be called with the same
// arguments.
Added initial send-to-backpack-request.js with tests.