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/main.c

@@ -0,0 +1,9 @@
+#include "path_test.h"
+#include "test.h"
+
+int main()
+{
+  testPath();
+  
+  return showTestResults();
+}