Add an iterator protocol

[Wiguwbe]

For for i,a in enumerate(iterable): cases

[rainwoodman]

Just realized this is a duplicate of #8.

Not only for enumerate(iterable), but also for coroutines (because tinypy doesn't support threads) -- to actually use 'yield' to yield the control flow to another context. coroutine is a field that I am not super familiar with, yet I feel it is very important if we want this language to be relevant in, for example, low-latency services running on embedded systems.