`apply-gen' now generates functions for partial applications

This is intended as a convenient shorthand:

```xml
  <!-- these two are identical -->
  <sequence select="f:apply( my:func(), $x, $y )" />
  <sequence select="my:func( $x, $y )" />
```
Short-hand partial application example added to README.md
`apply-gen' now ingores arity-overloaded functions

Rationale is provided in `apply-gen.xsl'.
`apply-gen' partial functions now generated

This produces a short-hand partial application (well, it'd be more accurate
to call the use of `f:apply' or `f:partial' long-hand, since this is most
natural).