Implement TEAL 4 basic ops

[jasonpaulos]

This PR adds support for the following feature from TEAL v4:

• extra_program_pages transaction field
• GeneratedID expression
• BitLen expression
• Sqrt expression
• ShiftLeft, ShiftRight, and Exp expressions
  • Also overloads the Python operators for these operations

[jasonpaulos]

@jdtzmn does GeneratedID(txnIndex) make sense for the gaid(s) opcode?

[jdtzmn]

Yes, I think that makes sense. @jannotti any thoughts?

[jannotti]

I'm ambivalent - I don't like that it implies the g is for "generated", as we're going for "group", as with gload and gtxn. But I guess you won't need the g's because it is known from context in pyteal? How will gload appear? Does the g in gtxn show up in pyteal naming?

I do like 'generated' compared to 'created' for being when clearer that it's about the ID that was made in that transaction.

So... I think it's good if you feel it fits in well with other choices in pyteal. I don't know it all that well.

[jdtzmn]

Looks good! I like the overloads.

[jasonpaulos]

@jannotti gload will appear in a separate PR after some design work -- I want to explore the idea of having Export/Import operations that would allow contracts to share values using gload.

And yes g does show up in the Gtxn variable, but that's currently the only place. I think I'll merge this with GeneratedID because I believe it's accurate enough, but if we change our mind about that name we'll have some time to update it before PyTeal gets released.

[jasonpaulos]

Closes #65.