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

src/bin/wasmtime.rs

@@ -46,7 +46,6 @@ use wasmtime_wasi::create_wasi_instance;
 use wasmtime_wasi::old::snapshot_0::create_wasi_instance as create_wasi_instance_snapshot_0;
 #[cfg(feature = "wasi-c")]
 use wasmtime_wasi_c::instantiate_wasi_c;
-use wasmtime_wast::instantiate_spectest;
 
 const USAGE: &str = "
 Wasm runner.
@@ -275,12 +274,6 @@ fn main() -> Result<()> {
 
     let mut module_registry = HashMap::new();
 
-    // Make spectest available by default.
-    module_registry.insert(
-        "spectest".to_owned(),
-        HostRef::new(Instance::from_handle(&store, instantiate_spectest()?)),
-    );
-
     // Make wasi available by default.
     let preopen_dirs = compute_preopen_dirs(&args.flag_dir, &args.flag_mapdir);
     let argv = compute_argv(&args.arg_file, &args.arg_arg);