Store the match vars in mg_len instead of calling atoi() on mg_ptr.

The match variables $1, $2 etc, along with many other special scalars, have
magic type PERL_MAGIC_sv, with the variable's name stored in mg_ptr. The
look up in mg.c involved calling atoi() on the string in mg_ptr to get the
capture buffer as an integer, which is passed to the regex API.

To avoid this repeated use of atoi() at runtime, change the storage in the
MAGIC structure for $1, $2 etc and $&. Set mg_ptr to NULL, and store the
capture buffer in mg_len. Other code which manipulates magic ignores mg_len
if mg_ptr is NULL, so this representation does not require changes outside
of the routines which set up, read and write these variables.
(Perl_gv_fetchpvn_flags(), Perl_magic_get() and Perl_magic_set())
Store all other match vars in mg_len instead of mg_ptr/mg_len.

Perl_gv_fetchpvn_flags() now stores the appropriate RX_BUFF_IDX_* constant
in mg_len for $` $' ${^MATCH} ${^PREMATCH} and ${^POSTMATCH}
This makes some code in mg.c unreachable and hence unnecessary; the next
commit will remove it.
Remove now unused $` $' ${^MATCH} ${^PREMATCH} ${^POSTMATCH} code.

The previous commit's changes to Perl_gv_fetchpvn_flags() rendered this
code in Perl_magic_get() and Perl_magic_set() unreachable.
Simplify some code in Perl_magic_get() and Perl_magic_set().

Remove the checks that avoided confusing $^P, ${^PREMATCH} and ${^POSTMATCH}
now that the latter two do not take that code path. Remove a similar
check for $^S added by commit 4ffa73a366885f68 (Feb 2003). (This commit did
not add any other variable starting with a control-S.) This eliminates all
uses of the variable remaining.

Move the goto target do_numbuf_fetch inside the checks for PL_curpm, as
both its comefroms have already made the same check.
Add a perldelta entry for the changes to the internals of match variables.