array: reword chapter, show runtime failure
clone: focus on the Clone trait
constants: more details on the 'static lifetime and &'static str
iter: reduce length of the example output
vec: make clear that pop, returns the removed element
clone: rename chapter
methods: clarify what the argument self/&self/&mut self really means
option: show the output of the example
trait: update to latest Rust
enum: more idiomatic match, more comments. Closes #48
str: update description of a string. Closes #42
expression: move chapter to the staging area
Makefile: update whitelist