Abstract loading classes

[David Vernet]

We should have a separate service that loads classes in https://github.com/Decave/JCoz/blob/master/src/native/entry.cc#L272, and also exposes an interface for querying for whether a class is loaded, in scope, etc. The interface could look something like

loadClass(jvmtiEnv *jvmti_env, jclass klass)
classIsLoaded(jclass klass)
isClassInScope(jclass klass)