Implement context manager

[Wiguwbe]

To use with Something(): ...

[rainwoodman]

We should add this, but I do not see a clear path how this can be implemented, mostly due to my own limited experience on language and compilers.

1. We certainly need to update the tokenizer and the encoder such that the compiler can put this into a bytecode.
2. On the bytecode side, perhaps implement the with block as a try-except-finally structure. Note that we do not have a finally clause yet (I don't think we do).
3. We also need to support the object protocol, __enter__ and __exit__. Our exceptions are untyped, and we usually use 'str' objects. So this may couple with the type system which is also more or less hacked up.