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-wast/src/lib.rs

@@ -23,14 +23,12 @@
 )]
 
 extern crate alloc;
-#[macro_use]
-extern crate failure_derive;
 
 mod spectest;
 mod wast;
 
 pub use crate::spectest::instantiate_spectest;
-pub use crate::wast::{WastContext, WastError};
+pub use crate::wast::WastContext;
 
 /// Version number of this crate.
 pub const VERSION: &str = env!("CARGO_PKG_VERSION");