Add a lambda implementation

[TheRamenChef]

This pull request adds three new classes for a lambda implementation of Generator.

[mherrmann]

Thanks. Do you have some examples of their usage?

[TheRamenChef]

For example:

new LambdaGenerator<Integer>(g -> {
    g.yield(0);
    for (int i = 0; i < 10; i++) {
        g.yield(i);
        for (int j = 0; j < 10; j++) {
            g.yield(i + j);
            for (int k = 0; k < 10; k++)
                g.yield(i + j + k);
        }
    }
});