Expose wasmer::CraneliftOptLevel

This is required to do something like

```rust
use wasmer::{Cranelift, CraneliftOptLevel};

let mut compiler = Cranelift::default();
compiler.opt_level(CraneliftOptLevel::None);
let engine = JIT::new(compiler).engine();
// ...
```

lib/api/src/lib.rs

@@ -355,7 +355,7 @@ let store = Store::new(&engine);
 pub use wasmer_compiler_singlepass::Singlepass;
 
 #[cfg(feature = "cranelift")]
-pub use wasmer_compiler_cranelift::Cranelift;
+pub use wasmer_compiler_cranelift::{Cranelift, CraneliftOptLevel};
 
 #[cfg(feature = "llvm")]
 pub use wasmer_compiler_llvm::LLVM;