a job for someone? SvREFCNT_dec_NN()

I've just added the SvREFCNT_dec_NN() macro, which can be used in
situations where we know the sv isn't null: so that the check can be
skipped.

On gcc on x64_64 platforms, this makes each use of SvREFCNT_dec() 5 code
bytes shorter, and avoids a test and jump.

There are over 500 uses of SvREFCNT_dec() within the perl core; ideally
one or more volunteers could audit all those uses, and change them to the
_NN variant where its clear that the sv can't be null (e.g. because fields
within it have already been accessed, or it's an arg declared NN in
embed.fnc).

-- 
I don't want to achieve immortality through my work...
I want to achieve it through not dying.
    -- Woody Allen

• a job for someone? SvREFCNT_dec_NN() by Dave Mitchell
• Re: a job for someone? SvREFCNT_dec_NN() by Steffen Mueller
• Re: a job for someone? SvREFCNT_dec_NN() by Father Chrysostomos