296 Allow default-namespace=##any

[michaelhkay]

Allows the default namespace for elements and types to have the special value "##any", which causes unprefixed QNames to match elements in any namespace. Use cases include:

• Casual ad-hoc XPath queries, where over-retrieval isn't a problem
• Use with HTML, where it can be unpredictable whether elements will be in a namespace, and where users are accustomed to browser behaviour with its "wilful violation" of the XPath 1.0 specification
• Any environment where multiple namespaces are in use for variants of what is essentially the same vocabulary of element names (for example, where the XML designer has made the mistake of versioning the namespace URI)

[michaelhkay]

Fix #296

[ChristianGruen]

While it’s unlikely that ##any is already widely used in our solar system, a note would be helpful that explains the rationale behind this particular string.

At least for XQuery, explicit syntax feels safer to me (to avoid typos such as #any):

(: which, I assume, will return the element :)
declare default element namespace any;
<xml:x/>/self::x

In proprietary interfaces, null, undefined or anything else could be supplied to indicate this special case.

[michaelhkay]

Fix #379

[ndw]

The CG agreed to merge this PR at the Prague f2f.

[ndw]

The CG agreed to merge this PR without further discussion at meeting 81.