Make ly:make-unpure-pure-container accept a single callback (issue 7382046)

[mtsolo]

LGTM, with one suggestion: the difference between unpure and pure
functions in LilyPond is that unpure functions accept n arguments
whereas pure functions accept n+2. This shortcut you're proposing only
works when n=1, which you document, but it'd be nice if it worked for an
arbitrary number of input arguments.  It'd only be possible to do this,
though, if there were a way in C++ to determine the number of arguments
a Scheme function took.

https://codereview.appspot.com/7382046/