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/misc_testsuite/stack_overflow.wast

@@ -0,0 +1,26 @@
+(module
+  (func $foo
+    (call $foo)
+  )
+  (func (export "stack_overflow")
+    (call $foo)
+  )
+)
+
+(assert_exhaustion (invoke "stack_overflow") "call stack exhausted")
+(assert_exhaustion (invoke "stack_overflow") "call stack exhausted")
+
+(module
+  (func $foo
+    (call $bar)
+  )
+  (func $bar
+    (call $foo)
+  )
+  (func (export "stack_overflow")
+    (call $foo)
+  )
+)
+
+(assert_exhaustion (invoke "stack_overflow") "call stack exhausted")
+(assert_exhaustion (invoke "stack_overflow") "call stack exhausted")