Proposals for variadic functions

Hi,

I've gone ahead and written up a series of proposals with the intention of
splitting up the variadic functions work into more manageable chunks that
can be reviewed and added to the standard.

I've not added any standard text or syntax to these yet. I've only provided
a high-level definition, motivating examples, and some additional notes.

Optional Parameters and Bounded Variadic Functions

1. Proposal to support optional parameter values on static functions
<https://github.com/qt4cg/qtspecs/issues/155>.
2. Proposal to support optional parameters that bind to the context item.
<https://github.com/qt4cg/qtspecs/issues/157>
3. Proposal to support optional parameters on dynamic functions.
<https://github.com/qt4cg/qtspecs/issues/158>

NOTE: 1 and 2 should be sufficient for Dimitre's suggestion for using this
mechanism to simplify the function signatures in the XSLT and XQFO
specifications.

Named/keyword arguments

1. Proposal to support named arguments on static function calls
<https://github.com/qt4cg/qtspecs/issues/159>.
2. Proposal to support named arguments on dynamic function calls.
<https://github.com/qt4cg/qtspecs/issues/160>

Variadic Parameters

1. Proposal to support unbounded variadic functions on sequence parameters.
<https://github.com/qt4cg/qtspecs/issues/161>
2. Proposal to support unbounded variadic functions on map parameter keys.
<https://github.com/qt4cg/qtspecs/issues/162>

NOTE: I think this covers all the discussed functionality around variadic
functions. If not, I'm happy to create other issues to base discussions
around.

Kind regards,
Reece