make overload respect get magic

make overload respect get magic

In most places, ops checked their args for overload *before* doing
mg_get(). This meant that, among other issues, tied vars that
returned overloaded objects wouldn't trigger calling the
overloaded method.  (Actually, for tied and arrays and hashes, it
still often would since mg_get gets called beforehand in rvalue
context).

This patch does the following:

Makes sure get magic is called first.

Moves most of the overload code formerly included by macros at the
start of each pp function into the separate helper functions
Perl_try_amagic_bin, Perl_try_amagic_un, S_try_amagic_ftest,
with 3 new wrapper macros:
tryAMAGICbin_MG, tryAMAGICun_MG, tryAMAGICftest_MG.
This made the code 3800 bytes smaller.

Makes sure that FETCH is not called multiple times. Much of this
bit was helped by some earlier work from Father Chrysostomos.

Added new functions and macros sv_inc_nomg(), sv_dec_nomg(),
dPOPnv_nomg, dPOPXiirl_ul_nomg, dPOPTOPnnrl_nomg, dPOPTOPiirl_ul_nomg
dPOPTOPiirl_nomg, SvIV_please_nomg, SvNV_nomg (again, some of
these were based on Father Chrysostomos's work).

Fixed the list version of the repeat operator (x): it now only
calls overloaded methods for the scalar version:
    (1,2,$overloaded) x 10
no longer erroneously calls
    x_method($overloaded,10))

The only thing I haven't checked/fixed yet is overloading the
iterator operator, <>.