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/environ/src/data_structures.rs

@@ -9,7 +9,7 @@ pub mod ir {
 }
 
 pub mod settings {
-    pub use cranelift_codegen::settings::{builder, Configurable, Flags};
+    pub use cranelift_codegen::settings::{builder, Builder, Configurable, Flags};
 }
 
 pub mod isa {