Remove the Flags type from Config API (#769)

* Remove the `Flags` type from `Config` API

This commit removes the final foreign type from the `Config` API in the
`wasmtime` crate. The cranelift `Flags` type is now expanded into
various options on the `Config` structure itself, all prefixed with
`cranelift_` since they're only relevant to the Cranelift backend. The
various changes here were:

* The `avoid_div_traps` feature is enabled by default since it seemed
  that was done anywhere anyway.
* Enabling the wasm SIMD feature enables the requisite features in
  Cranelift as well.
* A method for enabling the debug verifier has been added.
* A method for configuring the Cranelift optimization level, as well as
  a corresponding enumeration, has been added.

* Assert that `Config` is both `Send` and `Sync`

crates/api/src/lib.rs

@@ -27,7 +27,7 @@ pub use crate::externals::*;
 pub use crate::instance::Instance;
 pub use crate::module::Module;
 pub use crate::r#ref::{AnyRef, HostInfo, HostRef};
-pub use crate::runtime::{Config, Engine, Store, Strategy};
+pub use crate::runtime::{Config, Engine, OptLevel, Store, Strategy};
 pub use crate::trap::{FrameInfo, Trap, TrapInfo};
 pub use crate::types::*;
 pub use crate::values::*;