Deprecate passing two arguments to Model.new

Model.call is used to create objects already backed by the
database, this just existed for backwards compatibility.
Add removal reminder markers to some associations methods
Add after_initialize plugin for the after_initialize hook

after_initialize isn't used much, but it causes a performance hit
for all model instantiation.  Moving it to a plugin makes that
performance hit only affect people who use the hook.

The hook_class_methods plugin will no longer support the
after_initialize hook at all.  Anyone adding after_initialize
hooks via hook_class_methods will need to switch to defining
an instance method.
Add Model#errors_class private method for choosing the errors class on a per-model basis
Deprecate Model::Errors#[] autovivification (returning empty array when missing)

Autovivification causes problems as it makes #[] not return a
falsely value.  Unfortunately, the active_model spec requires it,
so have just that plugin enable it.
Deprecate nil dataset options and modifying initial dataset

There isn't any reason to support nil as an argument, since the
default argument should be a hash.

The deprecation of modifying the initialize dataset is a
performance optimization, since it means that in Sequel 4, the
default dataset options hash can be a shared frozen hash.  As
clone is used for dataset modifications, none of the current specs
do this.

In the mutation methods, instead of modifying the @opts hash, just
reassign it to the new hash.  This should be faster, avoids
the future issues with a shared frozen hash, and is even more
correct if the opts for the clone have had entries deleted.
Deprecate Database#do as an alias to execute/execute_dui in some adapters

These are really old aliases, and inconsistent with the rest of
Sequel's usage.  Now that PostgreSQL supports Dataset#do to mean
something different, I don't like the inconsistency.

Database#query in the informix adapter conflicts with the query
extension.
Deprecate Dataset#set_{defaults,overrides}, move to set_overrides extension