Add initial std Symbol; Fix some type inference issues

std/assembly/symbol.ts

@@ -0,0 +1,27 @@
+import { Map } from "./map";
+
+var nextId: usize = 1;
+var stringToId: Map<string, usize>;
+var idToString: Map<usize, string>;
+
+export class Symbol {
+
+  static for(key: string): Symbol {
+    if (!stringToId) { stringToId = new Map(); idToString = new Map(); }
+    else if (stringToId.has(key)) return changetype<Symbol>(stringToId.get(key));
+    var id = nextId++;
+    stringToId.set(key, id);
+    idToString.set(id, key);
+    return changetype<Symbol>(id);
+  }
+
+  static keyFor(sym: Symbol): string | null {
+    return idToString !== null && idToString.has(changetype<usize>(sym))
+      ? idToString.get(changetype<usize>(sym))
+      : null;
+  }
+
+  constructor(description: string | null = null) {
+    return changetype<Symbol>(nextId++);
+  }
+}