make core safe against HvAUX() realloc

make core safe against HvAUX() realloc

Since the HvAUX structure is just tacked onto the end of the HvARRAY()
struct, code like this can do bad things:

    aux = HvAUX();
    ... something that might split hv ...
    aux->foo = ...; /* SEGV! */

So I've visually audited core for places where HbAUX() is saved and then
re-used, and re-initialised the var if it looks like HvARRAY() could
have changed in the meantime.

I've been very conservative about what might be unsafe. For example,
destructors or __WARN__ handlers could call perl code that modifies the
hash.