libcore: use unboxed closures in `float_to_str_bytes_common`
libcore: use unboxed closures in `slice::raw` free functions
libgetopts: use unboxed closures in `each_split_within`
libgraphviz: use unboxed closures in `LabelText` methods
librand: use unboxed closures in `distributions` module
libserialize: use unboxed closures
librbml: fix fallout
librbml: use unboxed closures in free functions
librustc: fix fallout