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/wast/Cargo.toml

@@ -11,12 +11,10 @@ readme = "README.md"
 edition = "2018"
 
 [dependencies]
-wasmtime-jit = { path = "../jit" }
-wasmtime-runtime = { path = "../runtime" }
-wasmtime-environ = { path = "../environ" }
-wast = "4.0.0"
 anyhow = "1.0.19"
-target-lexicon = "0.9.0"
+wasmtime = { path = "../api" }
+wasmtime-runtime = { path = "../runtime" }
+wast = "4.0.0"
 
 [badges]
 maintenance = { status = "actively-developed" }