Ensure mixin initializers are not overridden by subclasses.
Ensure mixin initializers are not overridden by subclasses.

Change visibility of mixin initializers to private. They are only used
by the specific class anyway, and are members of the class. This way, we
avoid them to be overridden by a subclass, which might also have mixins
with the same signature.

richard-roberts/SOMns,smarr/SOMns,richard-roberts/SOMns,richard-roberts/SOMns,smarr/SOMns,richard-roberts/SOMns,smarr/SOMns,richard-roberts/SOMns,smarr/SOMns,smarr/SOMns,richard-roberts/SOMns,smarr/SOMns,richard-roberts/SOMns,smarr/SOMns