RCR 208: Introduce freeze levels

Introduce freeze levels. The default level would function exactly as it does now by preventing any modifications to objects. By specifying an argument to the Object#freeze method, different levels of 'freezing' could be created.

Some Ruby users consider any changes to built-in Ruby classes/modules/objects to be dangerous - in general they can use the current 'freeze' method as-is to make sure that no changes can be made.
Other users feel that allowing changes to be made to classes/modules/objects to be a very beneficial feature of Ruby. However, they might feel that redefining built-in methods is a 'dangerous' practice.

The proposal is to introduce a new freeze level that would allow new methods to be defined for an object, but would not allow redefining already existing methods in the object.

This proposal would allow for an extra level of safety. Currently, freeze in an all-or-nothing proposition: if you freeze an object you can't add new methods to the object. The new freeze level would not allow the user to redefine methods on the object,but they would be able to add new ones.

An optional argument would be added to Object#freeze to indicate the new level.

  Object#freeze(level=0)
  #level 0 is the default
  #level 1 is the new level

Or, alternately, a new freeze method could be added to Object, perhaps called:

  Object#partial_freeze