Support logs, dynamic arg indexes, and other improvements

[jasonpaulos]

This PR adds:

• Support for logging with the Log expression
• Support for dynamic indexes into the Arg expression
• Support for single-element NaryExpr, e.g. And(Int(1))
• Support for creating Bytes expressions directly from bytes and bytearray objects, e.g. Bytes(b"value")

Closes #113.

[shiqizng]

it looks good to me. But I don't see why support for single-element naryExpr is needed. why is Arg(Int(1)) or Concat(Bytes("a")) useful?

[jasonpaulos]

Single element NaryExpr was a request from #80. The use case is to write something like And(*[validate(thing) for thing in things]) without worrying about a special case if the array has only 1 element.

[algochoi]

Code and tests look good to me 🚢
Just one small suggestion on formatting strings with f-strings if we are Python 3.6+.