Bump copyright year
Remove the _*_dataset_helper private method definition

This private method existed because when association support was
originally added to Sequel, instance_exec was not supported, so
the only way to pass arguments to an instance_evaled block was
to define a method for it.  Now that Sequel has dropped Ruby
1.8.6 support, all versions of ruby supported by Sequel support
instance_exec, so just use that instead.

The public AssociationReflection#dataset_helper_method is also
being removed, since it no longer has any use.
Optimize some #hash definitions

When Sequel supported ruby 1.8.6, it had to transform hashes to
sorted arrays inside #hash, since in ruby  1.8.6, Hash#hash is
different for two hashes even if they are equal according to ==.

Starting at ruby 1.8.7, Hash#hash is the same for hashes equal
according to ==, so we no longer need to create the sorted array,
which should make these #hash definitions faster.
Add core_refinements extension, providing refinement versions of Sequel's core extensions
Fix Database#copy_into on jdbc/postgres when an exception is raised

The previous implementation has a serious error in that if a ruby
exception was raised, it committed the copy instead of aborting
it.  The reason this wasn't caught by the specs is that it used
an internal transaction.  However, the method is not specced to
use an internal transaction, and the native postgres adapter does
not use an internal transaction, so remove the transaction usage.
In addition to aborting the copy correctly, also correctly wrap
the underlying exception in a DatabaseError.
Remove remaining use of meta_def from lib code
Remove meta_def usage from the model and adapter specs
Move the #meta_def support for Database, Dataset, and Model to the meta_def extension

This is a backwards incompatible change in that the #meta_def
method is no longer available by default on Database, Dataset,
and Model classes and instances.

Sequel originally depended on the metaid gem that added meta_def
and a handful of other methods to Object.  Over the years I removed
usage of most of the other methods, and restricted their definitions
in Sequel to just Database, Dataset, and Model classes and instances.
However, the metadef method was only used in a couple of places
in Sequel, and it really only saves a few characters in those
places, so I decided to remove usage of meta_def completely.

As meta_def was no longer used internally, and it was originally
only used for implementing Sequel, there was no need to load it
by default and have all Sequel users pay the memory penalty for
it.

Since some users may need it, I've decided to move it to an
extension to make it easy to work with old code.

The core and extension specs still use meta_def significantly, so
the extension is loaded automatically by those specs.  Hopefully
those specs can be cleaned up in the future.