Made Capstone an optional dependency (fixes #382) (#383)

* Made Capstone an optional dependency (fixes #382).

* Introduced feature 'disas' for disassembly (related to #382).

* Made 'disas' a default feature in cretonne-tools.

* Fixed errors in src/compile.rs introduced by get_disassembler changes.

- Moves `use` statements before the function declaration.
- Returns an error if the disassembler cannot be found created.

cranelift/src/cton-util.rs

@@ -17,6 +17,7 @@ extern crate docopt;
 extern crate filecheck;
 #[macro_use]
 extern crate serde_derive;
+#[cfg(feature = "disas")]
 extern crate capstone;
 extern crate term;