Add a "colocated" flag to symbol references. (#298)

This adds a "colocated" flag to function and symbolic global variables which
indicates that they are defined along with the current function, so they can
use PC-relative addressing.

This also changes the function decl syntax; the name now always precedes the
signature, and the "function" keyword is no longer included.

lib/wasm/src/environ/dummy.rs

@@ -182,7 +182,11 @@ impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environ
         // And maybe attempt some signature de-duplication.
         let signature = func.import_signature(self.vmctx_sig(sigidx));
         let name = get_func_name(index);
-        func.import_function(ir::ExtFuncData { name, signature })
+        func.import_function(ir::ExtFuncData {
+            name,
+            signature,
+            colocated: false,
+        })
     }
 
     fn translate_call_indirect(