Working around GROOVY-6263.

Using category horribly breaks the invocation of private method when 'this' points to a subtype.

This problem manifested where we subtype CpsTransformer into SandboxCpsTransformer. I could have just worked around this problem on that class by making every private method protected, but this problem will happen during the entire groovy-cps execution, which means any complex workflow libraries that involve class hierarchy will be doomed.

So I opted for inserting these methods like how GroovyDefaultMethods get inserted, namely through MetaMethod