Add *c_void (*Void) conversion functions

[brson]

*c_void (or potentially *Void #4724) is my preferred hammer when I need to pass around some untyped pointers. Currently I use transmute to convert back and forth, but I might instead want something like:

impl *Void {
  static unsafe fn from_borrowed<T: Voidable>(&T);
  unsafe fn to_borrowed<T: Voidable>(self) -> &T;
}

etc. Writing Void::from_borrowed makes the code more obvious than transmute, since transmute can mean anything.

[catamorphism]

Nominating for milestone 3, feature-complete

[graydon]

accepted for feature-complete milestone

[thestinger]

You can now coerce borrowed pointers to raw pointers, and it's possible to make arbitrary casts of raw pointers with as. It's still very error-prone to convert back to a borrowed pointer, but lifetimes make that tricky.

[brson]

This doesn't particularly matter to me any more, and Void should maybe not exist.

[pnkfelix]

assigning P-low.

[pnkfelix]

actually, closing, since brson no longer cares about this.