Minor cleanups and comments in GenBCode
Comment summarizing the JVM spec for InnerClass / EnclosingMethod
Remove unnessecary check when computing InnerClass attribute flags

The final flag is computed correctly by javaFlags.
Pattern matching on ClassBType extracts the inernalName
Clean up and document some usages of flags in the backend

When re-assigning the owner of a symbol, the original owner at each
phase is now stored in the symbol table. Method `ownerAtCurrentPhase`
returns the owner at the current phase.
Support writing classfile of version 52
Documentation for isModuleClass