Better embedding API support (and tests!) for strings with null bytes.

- Test that a foreign method can return strings.
- Test that a foreign method can return a string with null bytes.
- Test wrenCall().
- Allow passing NULL for "v" to wrenCall().
- Allow "a" for passing an explicit length byte array to wrenCall().

test/api/call.wren

@@ -0,0 +1,45 @@
+class Api {
+  static noParams {
+    System.print("noParams")
+  }
+
+  static zero() {
+    System.print("zero")
+  }
+
+  static one(one) {
+    System.print("one " + one.toString)
+  }
+
+  static two(one, two) {
+    // Don't print null bytes.
+    if (two is String && two.bytes.contains(0)) {
+      two = two.bytes.toList
+    }
+
+    System.print("two " + one.toString + " " + two.toString)
+  }
+
+  static getValue(value) {
+    // Return a new value if we aren't given one.
+    if (value == null) return ["a", "b"]
+
+    // Otherwise print it.
+    System.print(value)
+  }
+
+  foreign static runTests()
+}
+
+Api.runTests()
+// expect: noParams
+// expect: zero
+// expect: one 1
+// expect: two 1 2
+
+// expect: two true false
+// expect: two 1.2 3.4
+// expect: two 3 4
+// expect: two string another
+// expect: two null [a, b]
+// expect: two str [98, 0, 121, 0, 116, 0, 101]