Allow usage of the builder pattern when constructing ClassDefinitions

[coderbot16]

Since many users will only want to use a few features of ClassDefinition at a time it would make more sense to allow usage of the builder pattern, even if only as a utility. It's hard to see what each parameter means when there are so many:

MemberHolder holder = entry.getValue();
classDefs.add(new ClassDefinition(name, Collections.emptySet(), holder.fields, holder.methods, new HashSet<>(holder.fields), Collections.emptySet(), Collections.emptySet()));

versus

MemberHolder holder = entry.getValue();
ClassDefinition definition = new ClassDefinition.Builder(name)
	.publicizeFields(holder.fields)
	.publicizeMethods(holder.methods)
	.mutableFields(holder.fields)
	.build();