Add builder-style annotation creation method

[thecodewarrior]

Currently the two annotation creation methods accessible from java take pairs and a map as parameters. A much more convenient version could be done with a builder style:

Contract contractAnnotation = Mirror.newAnnotation(Contract.class,
    new Pair<>("value", "_, null -> null"),
    new Pair<>("pure", true)
);
// vs
Contract contractAnnotation = Mirror.newAnnotation(Contract.class)
    .set("value", "_, null -> null")
    .set("pure", true)
    .build();