Add builtin::numify

[leonerd]

Adds a new builtin function, builtin::numify, the number equivalent of builtin::stringify.

While here also remembers to add a test and documentation on how stringify handles undef. No behavioural change to stringify itself.

[tonycoz]

You need to SvGETMAGIC() before checking the flags:

$ ./perl ../21982.pl 
Built-in function 'builtin::numify' is experimental at ../21982.pl line 9.
Use of uninitialized value $magic in number conversion at ../21982.pl line 9.
0
123
$ cat ../21982.pl
#!perl
use v5.36.0;

my $val;

tie my $magic, "Foo";

$val = "123";
say builtin::numify($magic);
say $magic;

package Foo {
  sub TIESCALAR { bless {}, shift; }


  sub FETCH { $val }
}

Similarly:

$ ./perl -Ilib -E '$! = 0; open my $fh, "<", "unknown"; say builtin::numify($!); say $!+0'
Built-in function 'builtin::numify' is experimental at -e line 1.
0
2

[tonycoz]

Overloaded 0+ that returns a non-integer:

$ ./perl -Ilib ../21982b.pl 
Built-in function 'builtin::numify' is experimental at ../21982b.pl line 11.
3.14159265
3
$ cat ../21982b.pl
#!perl
use v5.36.0;
package Foo {
  use overload
    '0+' => sub { 3.14159265 },
    fallback => 1;
}
my $x = bless {}, "Foo";

say 0+$x;
say builtin::numify($x);

[tonycoz]

This one I'm not so sure about:

$ ./perl -Ilib ../21982c.pl 
Built-in function 'builtin::numify' is experimental at ../21982c.pl line 7.
3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679
3
$ cat ../21982c.pl
#!perl
use v5.36.0;
use bigfloat;
my $x = 3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679;

say 0+$x;
say builtin::numify($x);