[plt-scheme] 301.7

MzScheme and MrEd are now version 301.7 in the SVN repository trunk.

The change is minor and mostly internal. MzScheme's C API now includes
scheme_make_prim_closure_w_arity(), which is like
scheme_make_closed_prim_w_arity(), but with a slightly different
calling convention: the closure data is stored inline in the procedure
object, and the procedure object (which includes the closure data) is
passed as the last argument to the procedure's C function, instead of
passing the closure data as the first argument to the C function. This
change to the calling convention avoids a branch in the interpreter,
and it allows calls to "primitive closures" to use a fast path in JITed
code.

Matthew