Unicode Stability Policies Updated

The Unicode Character Encoding Stability Policies ensure that developers
know what they can depend on between successive releases of the Unicode
Standard.

Recent changes to these policies include new guarantees:

    * Property aliases will not be reused later for different properties.
    * Property value aliases will not be reused later for different
      property values.
    * Characters with the General_Category of Number are guaranteed to
      have a corresponding Numeric_Type value.

Additionally, the wording for two earlier guarantees about
General_Category and Bidi_Class have been clarified:

    * No new General_Category property values will ever be added.
    * New Bidi_Class property values can only be added for a tightly
      constrained class of new character additions.

For the exact wording of these new and updated guarantees, see Unicode
Character Encoding Stability Policies
<http://www.unicode.org/policies/stability_policy.html>.

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