Support inner transactions

[jasonpaulos]

Closes #112

In addition to supporting inner transactions, this PR also:

• Adds all TEAL 5 opcodes to the Op enum
• Adds new global and txn fields from TEAL 5
• Allows zero element Seq expressions (this is a no-op, but it's needed to support InnerTxnBuilder.SetFields({}))
• Enables dynamic array access, closes Dynamic Array Reference #109
• Consolidates all txn opcode tests (txn, gtxn, itxn)

[shiqizng]

this looks good to me. adding descriptions to the new classes, such as InnerTxnActionExpr, would be helpful. and what happened to the tests for gtxn/gtxnas?

[jasonpaulos]

I've just added documentation for inner transactions -- thank you for reminding me!

The existing tests for txn/gtxn were very repetitive, so instead of repeating them again for itxn, I got rid of all the handwritten tests in favor of the new method test_txn_fields in txn_test.py. This methods iterates through all possible fields and all the different txn objects to make sure they get converted properly into the IR layer.

[shiqizng]

looks good to me