Added parse() function to expression module

I added the parse() function to the expression module: 
http://www.w3.org/MarkUp/Forms/wiki/XPath_Expressions_Module#The_parse.28.29_Function

This was a Category:XForms20 feature: 
http://www.w3.org/MarkUp/Forms/wiki/Parse_and_serialize_functions

It was suggested to make it compatible with XPath 3.0 parse-xml(),  The 
result is compatible, not identical. Differences are explained in an 
editor note, which will be fodder for discussion.

Cheers,
John M. Boyer, Ph.D.
Distinguished Engineer, IBM Forms and Smarter Web Applications
IBM Canada Software Lab, Victoria
E-Mail: boyerj@ca.ibm.com 

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