Implement dup on unbound method

[eileencodes]

In Rails _read_attribute is defined on the top level ActiveRecord::Base rather than the subclasses. This results in object shapes not being able to use their cache. By implementing dup on unbound methods we can give each subclass a uniq iseq and unique inline cache, increasing our shape cache hits.

Note: experimental change that must be paired with changes to Active Record to see results.

[eregon]

I think it would be rather unexpected that UnboundMethod#dup copies the bytecode when e.g. UnboundMethod#clone doesn't and generally .dup/.clone are not deep, they are only shallow copies.
I think another method name would be warranted.

One way would be to class_eval that method definition in different subclasses to ensure copying the bytecode. That would avoid exposing such an implementation detail.

[eregon]

Ah sorry, this is Shopify/ruby, not ruby/ruby. I guess the experimental/draft PR is implicitly there then.