Obviate second argument of LY_DEFINE (Scheme function name)

LY_DEFINE was checking that the Scheme function name corresponded to the C++ name. This goes one step further and removes the second argument, letting it be computed automatically from the first. This is exactly what was already done with MAKE_SCHEME_CALLBACK.

The interesting parts to review are in lily/include/lily-guile-macros.hh and lily/function-documentation.cc.