Added "headers()" primitive.  Stub.
  New primitive: all_headers().

  This returns a table containing all header-names and values.

  NOTE:  Two issues with this function:

  * We couldn't call it `headers()` as we use a table with that name for display-purposes.
  * Using std::unordered map with UTFString causes compilation failure. This is a bug I didn't want to fight right now.