Add support for lazy builders

[tobyink]

This commit adds support for lazy builders:

use v5.12;

package Foo {
  use Class::XSAccessor
    constructor  => 'new',
    lazy_getters => { get_bar => 'bar' };

  sub _build_bar {
    my $self = shift;
    return 'iron bar';
  }
}

my $foo1 = Foo->new( bar => 'chocolate bar' );
say $foo1->get_bar;   # chocolate bar

my $foo2 = Foo->new;
say $foo2->get_bar;   # iron bar

When the lazy getter is called, if the hash key does not exist, a method called "build$slot" will be called to return a default value. This will be stored in the hash and then the getter will continue as normal.

Also lazy_accessors is supported, including support for chained => 1.

I think this patch could be very helpful for accelerating Moo-based code which tries to use Class::XSAccessor when possible, but currently cannot for any lazy-built attributes.

Dancer2 in particular makes heavy use of lazy-built attributes, so this patch has the potential to eventually speed up Dancer2-based websites.