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

lightbeam/Cargo.toml

@@ -26,7 +26,7 @@ typemap = "0.3"
 
 [dev-dependencies]
 lazy_static = "1.2"
-wabt = "0.9.2"
+wat = "1.0.2"
 quickcheck = "0.9.0"
 
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