256: Context for default function parameter expressions

[michaelhkay]

This is a renewed attempt to tackle issue 256, which concerns how to define the static and dynamic context for default value expressions for optional function parameters in XQuery and XSLT. The resolution is to define the static and dynamic context for these expressions in detail.

To make this work, some refinement of the static and dynamic context definitions is needed:

• default collation is moved from the static context to the dynamic context, with a note that it is always known statically except in the case when defining the default for a function parameter.
• static base URI (in the static context) and executable base URI (in the dynamic context) are now formally separated; previously we fudged this by saying they could be different, but without recognizing separate context components
• the base URI for resolving relative collation URIs is now implementation defined. This allows implementors to use either the compile-time or run-time base URI, or some other URI defined using a processor API.

[ndw]

This PR was accepted at meeting 037.