Created constraintDecl

[raffazizzi]

This is an initial implementation to address #2330. I would like some input on a couple of things.

• I made <anyElement/> optional in the content of constraintDecl. I think the element can be empty if all that's needed is specifying the query language binding.
• I allowed constraintDecl in encodingDesc to interleave with other elements. OK?
• schemaSpec has a <sequence> content model, so I couldn't interleave constraintDecl. Instead, I placed if after elements that typically go first (equiv, desc), but before spec / ref elements. Thoughts?
• I would appreciate feedback on the text I added to Chap 22 and whether it's in the right place.

[raffazizzi]

Note that this seems to result in relaxng that does not represent the ODD's model, but it may be a separate issue.

Expected:

<interleave>
  <choice>
    <oneOrMore>
      <ref name="model.encodingDescPart"/>  
    </oneOrMore>
    <oneOrMore>
      <ref name="model.pLike"/>
    </oneOrMore>
  </choice>
  <optional>
    <ref name="constraintDecl"/>
  </optional>
</interleave>

Actual:

<interleave>
   <oneOrMore>
      <choice>
         <ref name="model.encodingDescPart"/>
         <ref name="model.pLike"/>
      </choice>
   </oneOrMore>
   <optional>
      <ref name="constraintDecl"/>
   </optional>
</interleave>

[raffazizzi]

Closing as duplicate of #2594