Returning null on a few main functions.

...so it doesn't clutter the shell with module instance data.
Major refactoring to implement context-aware eval (fixes #5)

-CompiledFunction doesn't have `top_level_cfun` anymore. It can be
 inferred from `outter_cfun`. Also, its ctor no longer receives an
 environment or toplevel; the `load_body` function was updated
 accordingly. Finally, `asContext()` can receive either a stack frame or
 a variables dict -- no more environment or receiver. There are some
 wicked hacks in the prim code dealing with this.

-VMProcess and VMStackFrame were refactored. Implementation of the stack
 in the interpreter was leaking to their interfaces (ie, the top frame
 was not in the Process stack itself, but spreaded through its
 attributes: .locals, .r_*). This was fixed.

-idez was refactored based on the above and read-access to the context's
 vars during debugging is now implemented.
stepping on eval_do_var_def
Trying to improve (?) stack trace on meme-exceptions