These lambdas were marked mutable as they captured a Ptr wrapper
class by value, which then only returned const-qualified references
to the value they point from the previous const pointer operators.
Nothing is actually mutating in the lambdas state here, and now
that the Ptr operators don't add extra const qualifiers these
can be removed.
Derivatives of Core::Object should be constructed through
ClassName::construct(), to avoid handling ref-counted objects with
refcount zero. Fixing the visibility means that misuses like this are
more difficult.
