Fix Ant warnings

Ant complains about `includeantruntime` not being set.

    [javac] build.xml:136: warning: 'includeantruntime' was not set, defaulting to build.sysclasspath=last; set to false for repeatable builds

The Ant runtime was required for one javac, but not the rest.

Java complains unchecked operations.

    [javac] Note: Some input files use unchecked or unsafe operations.
    [javac] Note: Recompile with -Xlint:unchecked for details.

I added `-Xlint:unchecked` and then resolved all existing warnings.

I see. The JSON API is a little awkward; would be nice if it could give us either an Iterator or an Iterable, or something, but okay.