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/Cargo.toml

@@ -29,12 +29,13 @@ docopt = "1"
 serde = "1.0.8"
 serde_derive = "1.0.8"
 term = "0.5.1"
-capstone = "0.4"
+capstone = { version = "0.4", optional = true }
 wabt = { version = "0.3", optional = true }
 target-lexicon = "0.0.2"
 
 [features]
-default = ["wasm"]
+default = ["disas", "wasm"]
+disas = ["capstone"]
 wasm = ["wabt", "cretonne-wasm"]
 
 [workspace]