This is a copy-on-write proxy for frozen Ruby objects, so duplicating frozen
object is delayed until a method tries to change frozen object.

CowProxy classes for array, hash, string and struct are provided. Objects from
other classes will be proxied without copy-on-write, you have to create a
CowProxy class for them to enable copy-on-write. Also you have to create
CowProxy class for each object which have any getter method with arguments,
because default CowProxy won't wrap returned value. Immutable classes such as
Integer or TrueClass doesn't need copy-on-write proxy because they can't be
changed.

You can wrap every object in a proxy. Proxy will always send method calls to
wrapped object, and wrap returned value with a CowProxy class if method has no
argument, so a proxy will always return proxy objects for getters without
arguments. When a method tries to change a frozen object, if proxy has
copy-on-write enabled, it will duplicate frozen object and will send next method
calls to duplicated object, in other case an exception is raised.