Make PL_hints an alias for PL_compiling.cop_hints

PL_hints stores the hints at compile time that get copied into the
cop_hints field of each COP (in newSTATEOP).

Since perl-5.8.0-8053-gd5ec298, COPs have stored all the hints.

Before that, COPs used to store only some of the hints.  The hints
were copied here and there into PL_compiling, a static COP-shaped buf-
fer used during compilation, so that things like constant folding
would see the correct hints.  a0ed51b3 back in 1998 did that.

Now that COPs can store all the hints, we can just use
PL_compiling.cop_hints to avoid having to copy them from PL_hints from
time to time.

This simplifies the code and avoids creating bugs like those that
a547fd219 and 1c75beb82 fixed.
Stop -- from crashing on regexps

I mean dereferenced regexps, as returned by ${ qr// }.

It was creating a corrupt SV by setting the IOK flag on something
of type SVt_REGEXP.

This is something I broke when I stopped regexps from being POK
in perl 5.18.0.
Make ++ handle regexps and vstrings

$ ./perl -Ilib -e 'use Devel::Peek; $x = v97; ++$x; Dump $x'
SV = PVMG(0x7fbfa402b698) at 0x7fbfa402eed8
  REFCNT = 1
  FLAGS = (RMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x7fbfa3c066a8 "b"\0
  CUR = 1
  LEN = 24
  MAGIC = 0x7fbfa3c06348
    MG_VIRTUAL = 0
    MG_TYPE = PERL_MAGIC_vstring(V)
    MG_LEN = 3
    MG_PTR = 0x7fbfa3c13ee8 "v97"

The vstring magic is still attached (with something that does not
match the contents of the string), even after modifying it.  I
probably broke that in 5.18 after fixing it in 5.16, but I am too
lazy to check.

$ ./perl -le '$x = ${qr//}; $x++; print "$x"'
Assertion failed: (PL_valid_types_IVX[SvTYPE(_svivx) & SVt_MASK]), function Perl_sv_2pv_flags, file sv.c, line 2908.
Abort trap: 6

That I broke when I stopped regexps from being POK in perl 5.18.0.

It was creating a corrupt SV by setting the IOK flag on something of
type SVt_REGEXP.
Set PL_curcop to NULL in op.c:S_cop_free

Having PL_curcop pointing to a freed op is a Bad Idea and something
that has caused bugs in the past.

The intent of commit 1df5f7c195 was to do this, but this part of
the commit was accidentally omitted.

See <20130805200313.GS3729@plum.flirble.org>.
Restore NULL check to gv.c:newGP, removed by 19bad673

1df5f7c195 added checks to newGP to account for PL_curcop being NULL.
19bad673 inadvertently removed the NULL check for non-threaded builds,
except in one spot.

Since S_cop_free now sets PL_curcop to NULL as of the previous commit
(something 1df5f7c195 was meant to do but didn’t), this check is a
good idea, even though PL_curcop is never NULL here.

See also <20130805200313.GS3729@plum.flirble.org>.
gv.c:newGP: merge some threaded and non-threaded code

The previous commit cause the two alternate pieces of code to be
nearly identical.
gv.c:newGP: assert that PL_curcop is not NULL

It should never be null here, and we want to know if it is.  (It can
be set to null in op.c:S_cop_free, but it should always have been set
to something else by the time newGP is called.)

Nevertheless, we should leave the null checks in place, to avoid sabo-
taging non-debugging builds if there are any remaining bugs here.
Call get-magic before rejecting non-refs in sv_bless

Otherwise bless $tied_ref will die if FETCH hasn’t been called else-
where yet.