Adapt interpreter to the new VM runtime

The new runtime doesn't yet have the necessary support for
stdlib methods so, some core helpers are added temporarily
in order to run some code:

 - `v7_array_length` emulates accessing the `.length` prop on arrays
 - `v7_stringify_value|v7_to_json` render objects and arrays but doesn't
   invoke the `toString` method

Sometimes the allocated memory is freed, but generally the idea is
to defer to having a GC.
Merge pull request #82 from cesanta/interpreter

Adapt interpreter to the new VM runtime