Initial external decorator for annotating explicit import names; Use file name as default module name in imports; Emit empty memory if there are no static segments; Update TLSF and examples accordingly

tests/compiler/external.ts

@@ -0,0 +1,13 @@
+export declare function foo(): void; // external.foo
+export declare namespace foo {
+  export function bar(): void; // foo.bar
+}
+
+@external("bar")
+export declare function two(): void; // external.bar
+
+@external("foo", "baz")
+export declare function three(): void; // foo.baz
+
+@external("foo", "var")
+export declare const var_: i32;