Turing complete!

If, while statement and function are added(with break, continue, return
statement). The environment is working in similar way to python: you can
write to local env only, but you can read from global. Function are
stored as values in environment.
Closure

Function are properly implemented using closure. Environment is now
implemented using linked list style. If reading in current Env fails,
then try to read from parent environment. When writing to an variable,
write to the earliest occurence from current env to out most(global)
env. If no env contains the variable, then write to current env(local).