Improve internal representation of `abi.DynamicArray`

[jasonpaulos]

Currently the internal representation of dynamic ABI arrays is the same as its encoded form.

This internal representation can be improved, especially when the element type is static and has length 1. In this situation, it's completely unnecessary to store the 2 byte length prefix that's part of the official encoding.

[jannotti]

Would it be better to do this by introducing a new type like pt.Array for this purpose, since it is, as you say internal not part of the A B "Interface" to the outside world?