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

wasmtime-api/Cargo.toml

@@ -39,7 +39,6 @@ wasi-common = { git = "https://github.com/CraneStation/wasi-common", rev = "c3bf
 docopt = "1.0.1"
 serde = { "version" = "1.0.94", features = ["derive"] }
 pretty_env_logger = "0.3.0"
-wabt = "0.9.2"
 wasmtime-wast = { path="../wasmtime-wast" }
 wasmtime-wasi = { path="../wasmtime-wasi" }
 rayon = "1.1"