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/src/runtime.rs

@@ -94,7 +94,7 @@ impl Default for Config {
 
 #[derive(Default)]
 pub struct Engine {
-    config: Config,
+    pub(crate) config: Config,
 }
 
 impl Engine {