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/faerie/src/backend.rs

@@ -38,7 +38,7 @@ pub struct FaerieBuilder {
 impl FaerieBuilder {
     /// Create a new `FaerieBuilder` using the given Cranelift target, that
     /// can be passed to
-    /// [`Module::new`](cranelift_module/struct.Module.html#method.new].
+    /// [`Module::new`](cranelift_module::Module::new)
     ///
     /// Faerie output requires that TargetIsa have PIC (Position Independent Code) enabled.
     ///