SchemaInferenceStack is missing typedef support

SchemaInferenceStack.resolvePathExpression() in a situation like this:

module foo {
  prefix foo;
  namespace foo;

  typedef foo {
    type leafref {
      path /bar;
    }
  }

  leaf bar {
    type int64;
  }
}

The problem is that that there is no way to logically enter 'typedef foo', hence the stack can only remain uninitialized – and therefore it does not have a default namespace. That in turn means the path cannot be properly bound, leading to an IllegalArgumentException.

Introduce an 'enterTypedef()' method, which will give proper context to the resolution process.