Added support for isize/usize in the CastPass lint pass.

Extracted the match that determines an integer types's size in a
utility function and implemented support for usize/isize.
Added a needed feature to the crate root.
Added some tests to cover those cases, and a test I previously forgot.
Silenced two errors signaled by dogfood.sh in unicode.rs.

src/unicode.rs

@@ -40,6 +40,7 @@ fn check_str(cx: &Context, string: &str, span: Span) {
     }
 }
 
+#[allow(cast_possible_truncation)]
 fn str_pos_lint(cx: &Context, lint: &'static Lint, span: Span, index: usize, msg: &str) {
     span_lint(cx, lint, Span { lo: span.lo + BytePos((1 + index) as u32),
                                hi: span.lo + BytePos((1 + index) as u32),