Add definition of a fat pointer

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

Author: Samuel Wilson

src/appendix-glossary.md

@@ -16,6 +16,7 @@ cx                      |  we tend to use "cx" as an abbrevation for context. Se
 DAG                     |  a directed acyclic graph is used during compilation to keep track of dependencies between queries. ([see more](incremental-compilation.html))
 data-flow analysis      |  a static analysis that figures out what properties are true at each point in the control-flow of a program; see [the background chapter for more](./appendix-background.html#dataflow)  
 DefId                   |  an index identifying a definition (see `librustc/hir/def_id.rs`). Uniquely identifies a `DefPath`.
+Fat pointer             |  a two word value carrying the address of some value, along with some further information necessary to put the value to use. For example: a reference to a slice is a fat pointer, carrying the starting address of the slice and its length.
 free variable           |  a "free variable" is one that is not bound within an expression or term; see [the background chapter for more](./appendix-background.html#free-vs-bound)
 'gcx                    |  the lifetime of the global arena ([see more](ty.html))
 generics                |  the set of generic type parameters defined on a type or item