Answer the class that can be used to compile parse trees,
or nil if there is none (as is the case now). Not
used for methods if parserClass answers nil, and for
doits if evaluatorClass answers nil.
debuggerClass
Answer which class is to be used to debug a chain of contexts which
includes the receiver. nil means 'do not debug'; other classes are
sent #debuggingPriority and the one with the highest priority is
picked.
decompilerClass
Answer the class that can be used to decompile methods,
or nil if there is none (as is the case now).
evaluatorClass
Answer the class that can be used to evaluate doits,
or nil if there is none (as is the case now).
parserClass
Answer the class that can be used to parse methods,
or nil if there is none (as is the case now).