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/src/lib.rs

@@ -0,0 +1,34 @@
+//! JIT-style runtime for WebAssembly using Cranelift.
+
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
+#![warn(unused_import_braces)]
+#![deny(unstable_features)]
+#![cfg_attr(
+    feature = "clippy",
+    plugin(clippy(conf_file = "../../clippy.toml"))
+)]
+#![cfg_attr(
+    feature = "cargo-clippy",
+    allow(new_without_default, new_without_default_derive)
+)]
+#![cfg_attr(
+    feature = "cargo-clippy",
+    warn(
+        float_arithmetic,
+        mut_mut,
+        nonminimal_bool,
+        option_map_unwrap_or,
+        option_map_unwrap_or_else,
+        print_stdout,
+        unicode_not_nfc,
+        use_self
+    )
+)]
+
+extern crate cranelift_codegen;
+extern crate wabt;
+extern crate wasmtime_execute;
+
+mod wast;
+
+pub use wast::{wast_buffer, wast_file};