Start a wast testing harness and add some tests.

This implements a minimal wast testing harness in tests/wast.rs, which
runs the wast tests under tests/wast.

It also adds tests for trapping in a variety of ways, and fixes several
bugs exposed by those tests.

lib/environ/src/lib.rs

@@ -45,7 +45,9 @@ mod module;
 mod tunables;
 mod vmcontext;
 
-pub use compilation::{compile_module, Compilation, Relocation, RelocationTarget, Relocations};
+pub use compilation::{
+    compile_module, Compilation, RelocSink, Relocation, RelocationTarget, Relocations,
+};
 pub use environ::{ModuleEnvironment, ModuleTranslation};
 pub use module::{DataInitializer, Export, MemoryPlan, MemoryStyle, Module, TableElements};
 pub use tunables::Tunables;