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/tests/wast_files.rs

@@ -0,0 +1,22 @@
+extern crate cranelift_codegen;
+extern crate cranelift_native;
+extern crate wasmtime_wast;
+
+use cranelift_codegen::isa;
+use cranelift_codegen::settings;
+use cranelift_codegen::settings::Configurable;
+use std::path::Path;
+use wasmtime_wast::wast_file;
+
+include!(concat!(env!("OUT_DIR"), "/run_wast_files.rs"));
+
+#[cfg(test)]
+fn native_isa() -> Box<isa::TargetIsa> {
+    let mut flag_builder = settings::builder();
+    flag_builder.enable("enable_verifier").unwrap();
+
+    let isa_builder = cranelift_native::builder().unwrap_or_else(|_| {
+        panic!("host machine is not a supported target");
+    });
+    isa_builder.finish(settings::Flags::new(flag_builder))
+}