Remove unused imports

[theron-wang]

Removes unused imports in all output files.

Take, for example, ManyImports in the included test case.
Previously:

package com.example;

import java.util.List;
import java.io.*;
import static java.util.Map.*;
import static java.lang.Math.sqrt;
import static java.lang.Math.PI;

public class ManyImports {
}

Now:

package com.example;

public class ManyImports {
}

Even if the import type is a used type element, it should still be removed if the current, minimized file does not contain any instances.

[theron-wang]

@kelloggm No rush on the review, just whenever you're back and have time!

[kelloggm]

Overall this is a great set of changes. Once my minor comments are addressed, this can be merged without another review.

[theron-wang]

Changes made; thanks.