Rename convert to convert_with_precedence

It is bad style to have an interface method the same name as an
internal method with a different signature. Then on overriding one of
these methods the compiler only finds the overloaded one with the wrong
signature, doesn't look up the base class and fails.

We fix this problem by renaming the protected method. This simplifies
the code in all derived classes.

# Conflicts:
#	src/java_bytecode/expr2java.h

Author: Owen Jones