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/frontend/src/frontend.rs

@@ -96,7 +96,7 @@ impl FunctionBuilderContext {
     }
 }
 
-/// Implementation of the [`InstBuilder`](../codegen/ir/builder/trait.InstBuilder.html) that has
+/// Implementation of the [`InstBuilder`](cranelift_codegen::ir::InstBuilder) that has
 /// one convenience method per Cranelift IR instruction.
 pub struct FuncInstBuilder<'short, 'long: 'short> {
     builder: &'short mut FunctionBuilder<'long>,
@@ -207,7 +207,7 @@ impl<'short, 'long> InstBuilderBase<'short> for FuncInstBuilder<'short, 'long> {
 /// modifies with the information stored in the mutable borrowed
 /// [`FunctionBuilderContext`](struct.FunctionBuilderContext.html). The function passed in
 /// argument should be newly created with
-/// [`Function::with_name_signature()`](../function/struct.Function.html), whereas the
+/// [`Function::with_name_signature()`](Function::with_name_signature), whereas the
 /// `FunctionBuilderContext` can be kept as is between two function translations.
 ///
 /// # Errors
@@ -391,7 +391,7 @@ impl<'a> FunctionBuilder<'a> {
         self.func.create_heap(data)
     }
 
-    /// Returns an object with the [`InstBuilder`](../codegen/ir/builder/trait.InstBuilder.html)
+    /// Returns an object with the [`InstBuilder`](cranelift_codegen::ir::InstBuilder)
     /// trait that allows to conveniently append an instruction to the current `Ebb` being built.
     pub fn ins<'short>(&'short mut self) -> FuncInstBuilder<'short, 'a> {
         let ebb = self