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/test-programs/tests/wasm_tests/runtime.rs

@@ -1,21 +1,10 @@
 use anyhow::{bail, Context};
 use std::fs::File;
 use std::path::Path;
-use wasmtime::{Config, Engine, HostRef, Instance, Module, Store};
-use wasmtime_environ::settings::{self, Configurable};
+use wasmtime::{Engine, HostRef, Instance, Module, Store};
 
 pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> anyhow::Result<()> {
-    // Prepare runtime
-    let mut flag_builder = settings::builder();
-
-    // Enable proper trap for division
-    flag_builder
-        .enable("avoid_div_traps")
-        .context("error while enabling proper division trap")?;
-
-    let mut config = Config::new();
-    config.flags(settings::Flags::new(flag_builder));
-    let engine = Engine::new(&config);
+    let engine = Engine::default();
     let store = HostRef::new(Store::new(&engine));
 
     let global_exports = store.borrow().global_exports().clone();