Allow define-*-function to accept currying definitions

Allow define-*-function to accept currying definitions

This is actually only useful for define-scheme-function since the
purpose of a currying definition is to return a function rather than a
music expression.  A typical usage case would be a function creating
an engraver closure:

dia-engraver =
   (ly:pitch? ly:pitch?)
     (let ((store '()) (cause #f))
       (make-engraver

[...]

   \new Voice \with { \consists \dia-engraver c' g'' }

[...]