Add Trampoline::flatMap

[gdejohn]

As suggested by @jon-hanson.

• http://blog.higher-order.com/assets/trampolines.pdf
• https://github.com/functionaljava/functionaljava/blob/series/5.x/core/src/main/java/fj/control/Trampoline.java
• https://github.com/typemeta/funcj/blob/master/core/src/main/java/org/typemeta/funcj/control/Trampoline.java

That would apparently allow for things like this:

static Trampoline<Integer> fibonacci(int n) {
    if (n <= 1) {
        return terminate(n);
    } else {
        return call(() -> fibonacci(n - 1)).flatMap(
            x -> call(() -> fibonacci(n - 2)).flatMap(
                y -> terminate(x + y)
            )
        );
    }
}