Add a minimal path manipulation C module.

This is just for the VM's own internal use, for resolving relative
imports.

Also added a tiny unit test framework for writing tests of low-level
C functionality that isn't exposed directly by the language or VM.

test/unit/test.c

@@ -0,0 +1,29 @@
+#include <stdio.h>
+
+#include "test.h"
+
+int passes = 0;
+int failures = 0;
+
+void pass()
+{
+  passes++;
+}
+
+void fail()
+{
+  failures++;
+}
+
+int showTestResults()
+{
+  if (failures > 0)
+  {
+    printf("%d out of %d tests failed. :(\n", failures, passes + failures);
+    return 1;
+  }
+  
+  printf("All %d tests passed!\n", passes + failures);
+  return 0;
+}
+