Update sample code to use `getopts::usage` in `print_usage`
Separate borrowck into its own crate and remove dead code as well.
auto merge of #19695 : arthurtw/rust/master, r=steveklabnik
Revamped testing guide
auto merge of #19627 : steveklabnik/rust/testing_guide, r=cmr
auto merge of #19582 : nikomatsakis/rust/crateification, r=alexcrichton

r? @alexcrichton