LDML: a new syntax to reuse sort order from another 8bit simple collation

We'll add a new XML syntax to reuse sort order from another collation:

<collation name="latin1_test_ci" id="300">

  <rules>

    <import source="latin1_swedish_ci"/>

  </rules>

</collation>

This task is about 8bit simple collations only.

The same syntax will however later be reused by other collation types (e.g. non-8bit or non-simple)

References:
http://cldr.unicode.org/development/development-process/design-proposals/collation-additions#TOC-Collation-Import