* 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.
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