Switch from wabt crate to wast (#434)

* Switch lightbeam from `wabt` to `wast`

Switch from a C++-based `*.wat` parser to a Rust-based parser

* Remove unneeded `wabt` dev-dependency from wasmtime-api

* Rewrite `wasmtime-wast` crate with `wast-parser`

This commit moves the `wasmtime-wast` crate off the `wabt` crate on to
the `wast-parser` crate which is a Rust implementation of a `*.wast` and
`*.wat` parser. The intention here is to continue to reduce the amount
of C++ required to build wasmtime!

* Use new `wat` and `wast` crate names

Cargo.toml

@@ -33,7 +33,7 @@ failure = "0.1"
 target-lexicon = { version = "0.8.1", default-features = false }
 pretty_env_logger = "0.3.0"
 file-per-thread-logger = "0.1.1"
-wabt = "0.9.2"
+wat = "1.0.2"
 libc = "0.2.60"
 rayon = "1.1"
 wasm-webidl-bindings = "0.5"