Add finally in case Thread.yield produces an exception.
Move Thread#exclusive into Java, to avoid Mutex dependency.
Depollute LOADED_FEATURES.

* enumerator.jar, complex.jar, rational.jar are now "provided" as
  faux already-required libraries.
* Do the same for jruby.jar and java.jar, standard since 1.7.
BUG is less scary than ERROR, and these aren't necessarily fatal.
Ensure dir exists before calling javac here.
Allow Symbols to GC.

This is a somewhat naïve change, but it appears to maintain a
stable memory profile during the following code:

```ruby
i = 0; loop { i.to_sym; i+=1 }
```

It also passes test:mri, test:jruby, and spec:ruby locally.

Could it really be this easy?