Add a more helpful error message when you haven't defined an allocator (#108)

Also adds a system for writing tests that assert that certain error codes are triggered so that I could test this.

tests/compiler/new-without-allocator.ts

@@ -0,0 +1,8 @@
+// Expect error: TS2304: Cannot find name 'allocate_memory'.
+// Expect error: AS214: An allocator must be declared to allocate memory. Try importing allocator/arena or allocator/tlsf.
+class A {}
+
+export function test(): i32 {
+  var a = new A();
+  return 3;
+}