Convert the wast test harness into a crate.

This uses a build.rs file to collect all the wast tests and create
individual `#[test]` lines for them, so that `cargo test` can run them
in parallel.

lib/wast/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "wasmtime-wast"
+version = "0.0.0"
+authors = ["The Cranelift Project Developers"]
+publish = false
+description = "wast testing support for wasmtime"
+categories = ["wasm"]
+repository = "https://github.com/CraneStation/wasmtime"
+license = "Apache-2.0 WITH LLVM-exception"
+readme = "README.md"
+
+[dependencies]
+cranelift-codegen = { git = "https://github.com/sunfishcode/cranelift.git", branch = "guard-offset" }
+cranelift-native = { git = "https://github.com/sunfishcode/cranelift.git", branch = "guard-offset" }
+wasmtime-execute = { path = "../execute" }
+wabt = "0.7"
+
+[badges]
+maintenance = { status = "experimental" }
+travis-ci = { repository = "CraneStation/wasmtime" }