Isolating Interpreter from Process.

Created a Process class with its own stack and registers.
Process objects hold a reference to the Interpreter instance.
Debugger working with step into.

Refactored Interpreter to work with VMProcess'es and used greenlet to
switch between co-routines. On "debug;" statement, a VMProcess for the
debugger is initiated.
debugger: implemented step over
debugger: implemented  continue
KeyError was being suppressed somewhere.

prim KeyError was being printed but execution continued. The unstable
state naturally caused other errors but the root problem was annoying to
spot.
debugger: update & rewind current function.

RewindException is used to rewind the current stack frame and recall the
function with the same parameters, register states and clean locals.
Fixed get_env wrongly mappping env_table and values
Merge branch 'dbg'