Log Message:
Update to MzScheme 4.2.4

Changes:

Version 4.2.4, January 2010
Added scheme/flonum and scheme/fixnum
Extended scheme/unsafe/ops
Changed JIT to support unboxed local binding of known-flonum
 arguments to unsafe-fl functions
Changed JIT to inline numeric ops with more than 2 arguments
Added #:save-errno option for foreign-function types
Added module->imports and module->exports

Version 4.2.3, November 2009
Changed _pointer (in scheme/foreign) to mean a pointer that does not
 refer to GCable memory; added _gcpointer
Added scheme/vector