Lexical stubs should not AUTOLOAD

There is a feature that allows stubs to fall back to their GVs’
CVs when called.  If I reference a stub, e.g., \&bar, and then
bar is autoloaded, the AUTOLOAD sub assigning *bar = *foo or
*bar = sub {...}, I can still call the stub to which I have a refer-
ence, and it will fall back to the overloaded sub.

That is all fine and dandy, but it causes any stub that references a
GV via its CvGV pointer to call that GV’s CV.  If we name a lexical
sub by pointing its CvGV pointer at the GV whose name we want it to
have, then the lexical sub, if undefined, will try to fall back to an
autoloaded sub.

That causes things to gang agley in cases like this:

    use 5.01;
    sub foo { } # package sub
    state sub foo;
    foo(); # calls lexical sub; falls back to package sub

While we could fix this by flagging the sub and checking for the flag
in pp_entersub (as we do with anonymous subs), it is better simply to
use a HEK, instead of a GV.  Since a GV is quite heavyweight for stor-
ing just a name, I was going to do that anyway, eventually.  Doing it
now fixes a bug.
op.c:newMYSUB: inline var used only once

as of the previous commit
op.c:newMYSUB: Remove unused vars
‘Undefined subroutine &foo called’ for lex subs

instead of just ‘Undefined subroutine called’ without the name.
Make pad_fixup_inner_anons cope with closed-over subs

When a sub starts being parsed, a new CV is created.  When it fin-
ishes, it is stored in its final location.  If there is a stub there
already, the pad is copied to the stub and the body attached thereto.

Since there may be closures inside the sub whose CvOUTSIDE
pointers point to the temporary CV used during compilation,
pad_fixup_inner_anons is called, to reassign all those
CvOUTSIDE pointers.

This happens in cases like this:

    sub f;
    sub f { sub { } }

When a sub closes over a lexical item in an outer sub, the inner sub
gets its own pad entry with the same value as the outer pad entry.

This means that, now that we have lexical subs (currently just state
subs), we can end up with a pad entry (&s) holding a sub whose
CvOUTSIDE does not point to the sub (f) that owns the pad:

    state sub s { }
    sub f { s() }

If the f sub has to reuse a stub, then pad_fixup_inner_anons gets to
see that, and complains bitterly:

$ ./perl -Ilib -E 'state sub s; sub f; sub f { s() }'
Assertion failed: (CvOUTSIDE(innercv) == old_cv), function Perl_pad_fixup_inner_anons, file pad.c, line 2095.
Abort trap
lexsub.t: Fix some tests

I got this working a few commits ago, but the tests mentioned the
wrong sub name.
Don’t say ‘variable &foo’ in warnings

It should be ‘subroutine &foo’.  (It could be ‘subroutine foo’, but we
use both forms elsewhere, and &foo is the easier to implement, the &
already being contained in the pad name.)
perldiag: closure referents → closure references

This goes back to 2ba9eb46.
Clone state subs in anon subs

Since state variables are not shared between closures, but only
between invocations of the same closure, state subs should behave
the same way.

This was a little tricky.  When we clone a sub, we now clone inner
state subs at the same time.  When walking through the pad, cloning
items, we cannot simply clone the inner sub when we see it, because it
may close over things we haven’t cloned yet:

    sub {
        state sub foo;
        my $x
        sub foo { $x }
    }

We can’t just delay cloning it and do it afterwards, because they may
be multiple subs closing over each other:

    sub {
       state sub foo;
       state sub bar;
       sub foo { \&bar }
       sub bar { \&foo }
    }

So *all* the entries in the new pad must be filled before any inner
subs can be cloned.

So what we do is put a stub in place of the cloned sub.   And then
in a second pass clone the inner subs, reusing the stubs from the
first pass.