Disconnects Store state fields from Compiler (#1761)

*  Moves CodeMemory, VMInterrupts and SignatureRegistry from Compiler
*  CompiledModule holds CodeMemory and GdbJitImageRegistration
*  Store keeps track of its JIT code
*  Makes "jit_int.rs" stuff Send+Sync
*  Adds the threads example.

examples/gcd.rs

@@ -11,8 +11,8 @@ fn main() -> Result<()> {
     // `Module` which is attached to a `Store` cache. After we've got that we
     // can instantiate it.
     let store = Store::default();
-    let module = Module::from_file(&store, "examples/gcd.wat")?;
-    let instance = Instance::new(&module, &[])?;
+    let module = Module::from_file(store.engine(), "examples/gcd.wat")?;
+    let instance = Instance::new(&store, &module, &[])?;
 
     // Invoke `gcd` export
     let gcd = instance