Document std::libc::c_void.
Add a `mut_split()` method for dividing one `&mut [T]` into two
auto merge of #7690 : kevinmehall/rust/document-c_void, r=huonw

I added documentation for when to use and not to use `c_void`, since it tripped me up when I started. (See issue #7627)
auto merge of #7691 : nikomatsakis/rust/vec-split-method, r=thestinger

I don't think we have this yet. This makes `&mut [T]` much more flexible.

r? @strcat (or whomever)