Make PL_hints an alias for PL_compiling.cop_hints

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.