auto merge of #5407 : jbclements/rust/add-assert-eq-macro, r=jbclements

Adds an assert_eq! macro that asserts that its two arguments are equal. Error messages can therefore be somewhat more informative than a simple assert, because the error message includes "expected" and "given" values.

diff --cc src/libstd/json.rs
Simple merge

diff --cc src/libsyntax/codemap.rs
Simple merge

diff --cc src/libsyntax/parse/lexer.rs
Simple merge

diff --cc src/libsyntax/print/pprust.rs
Simple merge