Fix broken links using rustdoc nightly

Uses cross-crate documentation links so that rustdoc does the hard work
of making relative links for us.

Requires nightly version of rustdoc in order to generate links based on
path names, see
https://github.com/rust-lang/rfcs/blob/master/text/1946-intra-rustdoc-links.md
for details.

cranelift/codegen/src/binemit/memorysink.rs

@@ -85,7 +85,7 @@ pub trait RelocSink {
 /// A trait for receiving trap codes and offsets.
 ///
 /// If you don't need information about possible traps, you can use the
-/// [`NullTrapSink`](binemit/trait.TrapSink.html) implementation.
+/// [`NullTrapSink`](NullTrapSink) implementation.
 pub trait TrapSink {
     /// Add trap information for a specific offset.
     fn trap(&mut self, _: CodeOffset, _: SourceLoc, _: TrapCode);