Slow java_package! with 200 classes

[fpoli]

I noticed that when there are a lot of classes (>200 in my case) the java_package procedural macro takes several tens of seconds to run. I suspect that this is because for each class duchess spawns a new javap process, which in turns runs a full JVM.

• Check if the bottleneck is really javap.
• If the bottleneck is javap, it should be possible to keep reusing the same JVM by using the Java interface of javap.

[fpoli]

The flamegraph of a cargo build confirms that 66% of the samples are in the JVM (start_thread). Within the JVM, 54% of the samples are in the Java compiler (CompileBroker::invoke_compiler_on_method).