Represent all templates as python modules, when compiling to python code

[riffm]

Now if an exception is raised in slot of inherited template there is no sane way to know the actual place (line number, source code).

That's sad.

So I propose to compile each template to python module. If a template has base template we just import all stuff from base-template-module at the top.

Benefits::

• natural "Inheritace" (slots overlaps over slots if there are any)
• each peace of compiled module code has corresponding lineno and col_offset in mint template
  and in traceback we see the right mint source code and template name