speed up (non)overloaded derefs

Consider a class that has some minimal overloading added - e.g. to give
pretty stringification of objects - but which *doesn't* overload
dereference methods such as '@[]'. '%[]' etc.

In this case, simple dereferencing, such as $obj->[0] or  $obj->{foo}
becomes much slower than if the object was blessed into a non-overloaded
class.

This is because every time a dereferencing is performed in pp_rv2av for
example, the "normal" code path has to go through the full checking of:

  * is the stash into which the referent is blessed overloaded? If so,
  * retrieve the overload magic from the stash;
  * check whether the overload method cache has been invalidated and if so
    rebuild it;
  * check whether we are in the scope of 'no overloading', and if so
    is the current method disabled in this scope?
  * Is there a '@{}' or whatever (or 'nomethod') method in the cache?
    If not, then process the ref as normal.

That's a lot of extra overhead to decide that an overloaded method doesn't
in fact need to be called.

This commit adds a new flag to the newish xhv_aux_flags field,
HvAUXf_NO_DEREF, which signals that the overloading of this stash
contains no deref (nor 'nomethod') overloaded methods. Thus a quick check
for this flag in the common case allows us to short-circuit all the above
checks except the first one.

Before this commit, a simple $obj->[0] was about 40-50% slower if the
class it was blessed into was overloaded (but didn't have deref methods);
after the commit, the slowdown is 0-10%. (These timings are very
approximate, given the vagaries of nano benchmarks.)

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