Fix diagnosis of undefined symbols (fix #56)

We were testing for `undefined` in our final check for whether the
symbol was a global, but were getting `ArkUndefined` as the return
result. A shame that TypeScript doesn't flag this up!

src/ark/basics.test.ts

@@ -14,6 +14,12 @@ import {compile} from './reader.js'
 import {testArkGroup as testGroup} from '../testutil.js'
 import {expToInst} from './flatten.js'
 
+test('Undefined symbol', (t) => {
+  const error = t.throws(() => new ArkState(expToInst(compile(['f']))).run())
+  t.not(error, undefined)
+  t.is(error.message, 'Undefined symbol f')
+})
+
 testGroup('Concrete values', [
   ['4', 4],
   ['["str","hello é"]', 'hello é'],

src/ark/reader.ts

@@ -14,6 +14,7 @@ import {
   ArkSet, ArkLocal, ArkCapture, ArkListLiteral, ArkObjectLiteral, ArkMapLiteral,
   ArkFn, ArkGenerator, ArkReturn, ArkYield,
   ArkProperty, ArkLet, ArkCall, ArkLiteral, ArkObject,
+  ArkUndefined,
 } from './interpreter.js'
 import {expToInst} from './flatten.js'
 
@@ -124,7 +125,7 @@ export function symRef(env: Environment, name: string): ArkLvalue {
   }
   // Finally, see if it's a global, and if not, error.
   if (lexp === undefined) {
-    if (env.externalSyms.get(name) === undefined) {
+    if (env.externalSyms.get(name) === ArkUndefined) {
       throw new ArkCompilerError(`Undefined symbol ${name}`)
     }
     lexp = new ArkProperty(new ArkLiteral(env.externalSyms), name)