serialisation - add FREEZE and THAW support

[shawnlaffan]

Serialisation using Sereal, and presumably other such systems, results in fatal errors. Reproducer below, along with its output.

Fixing this should (one hopes) just be a case of adding FREEZE and THAW methods, as documented at https://metacpan.org/pod/Sereal::Encoder#FREEZE%2FTHAW-CALLBACK-MECHANISM

Similar code has already been added for Storable via PDL::IO::Storable so it should be a case of reusing that.

I'll run some testing and if it works then I'll raise a PR.

#  reproducer
use strict;
use warnings;
use 5.014;
use PDL;
use Sereal qw/decode_sereal encode_sereal/;
my $x = decode_sereal encode_sereal (xvals(25,25));

say 'Result:';
say '=====';
say $x;
say '=====';
say 'Script completed';

Result:
=====
Stringizing problem: Fatal error: argument is probably not an ndarray, or magic no overwritten. You're in trouble, guv: 2453f26e5a8 245412ebf40 1111638597
 at C:/perls/5.40.0.1_PDL/perl/vendor/lib/PDL/Core.pm line 3294.
        PDL::string(PDL=SCALAR(0x2453f26e5a8), undef, "") called at serialise.pl line 9
        (in cleanup) Fatal error: argument is probably not an ndarray, or magic no overwritten. You're in trouble, guv: 2453f26e5a8 245412ebf40 1111638597