Update the *.wast runner to use the wasmtime API (#690)

* Update the `*.wast` runner to use the `wasmtime` API

This commit migrates the `wasmtime-wast` crate, which executes `*.wast`
test suites, to use the `wasmtime` crate exclusively instead of the raw
support provided by the `wasmtime-*` family of crates.

The primary motivation for this change is to use `*.wast` test to test
the support for interface types, but interface types is only being added
in the `wasmtime` crate for now rather than all throughout the core
crates. This means that without this transition it's much more difficult
to write tests for wasm interface types!

A secondary motivation for this is that it's testing the support we
provide to users through the `wasmtime` crate, since that's the
expectation of what most users would use rather than the raw
`wasmtime-*` crates.

* Run rustfmt

* Fix the multi example

* Handle v128 values in the `wasmtime` crate

Ensure that we allocate 128-bit stack slots instead of 64-bit stack
slots.

* Update to master

* Add comment

crates/api/examples/multi.rs

@@ -45,7 +45,12 @@ const WAT: &str = r#"
 fn main() -> Result<()> {
     // Initialize.
     println!("Initializing...");
-    let engine = HostRef::new(Engine::default());
+    let mut cfg = Config::new();
+    cfg.features(wasmtime_jit::Features {
+        multi_value: true,
+        ..Default::default()
+    });
+    let engine = HostRef::new(Engine::new(&cfg));
     let store = HostRef::new(Store::new(&engine));
 
     // Load binary.