Check that a method signature conforms to a functional interface

It would be useful to have an annotation like `@ConformsTo(Callable.class)` one can use on a method, which error-prone can check at compile time. Making sure that methods intended for use with Streams API as combiners or merge operators have the correct signature one possible use case; verifying signatures for methods intended for reflective use is another (admittedly minor) case.

It's probably best to have a way to indicate our intention about the receiver: `@ConformsTo(statically = AsyncFunction.class)` would be equivalent to `@ConformsTo(AsyncCallable.class)` on an (instance) method with with the signature `ListenableFuture doWork()`.

The annotations above are really straw-man examples; if you think this is worthwhile we can think about better alternatives.