Add definition of a fat pointer, double pointer, and wide pointer
Add definition of a fat pointer, double pointer, and wide pointer

Definition referenced from Programming Rust by Jim Blandy & Jason
Orendorff, published by O'Reilly. Page 214: References to Slices
and Trait Objects.

Double pointer and wide pointer both refer to fat pointer for
detail.

rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide