layout rtti with fixed ids for buffers and strings

std/assembly/shared/target.ts

@@ -0,0 +1,11 @@
+// This file is shared with the compiler and must remain portable
+
+/** Compilation target. */
+export enum Target {
+  /** WebAssembly with 32-bit pointers. */
+  WASM32,
+  /** WebAssembly with 64-bit pointers. Experimental and not supported by any runtime yet. */
+  WASM64,
+  /** Portable. */
+  JS
+}