Add a split-use pass.

It transforms "use std::io::{Read, Write}" into
"use std::io::Read; use std::io::Write"

Which later allows to remove just precisely the statements
we do not need, and leave rest be.

The test from the last commit does not pass, but that
is seemingly to do with the test harness setup, since it
works fine locally.

src/lib.rs

@@ -142,6 +142,7 @@ pub fn minimize(options: Options, stop: Arc<AtomicBool>) -> Result<()> {
 
     minimizer.run_passes([
         passes::EverybodyLoops::default().boxed(),
+        passes::SplitUse::default().boxed(),
         passes::FieldDeleter::default().boxed(),
         passes::Privatize::default().boxed(),
     ])?;