Operator overloading for Concat and Bytes

[PabloLION]

Problem

Bytes('a') + Bytes('b') will report type check error because of this: https://github.com/algorand/pyteal/blob/master/pyteal/ast/expr.py#L72-L73

pyteal/pyteal/ast/expr.py

Lines 72 to 73 in 1a1c93f

	def __add__(self, other):
	from pyteal.ast.naryexpr import Add

Solution

Bytes('a') + Bytes('b') returns Concat(Bytes('a'),Bytes('b')), or Bytes('ab')

Dependencies

Urgency

Ergonomic feature, not urgent.

[barnjamin]

dupe of #496

[PabloLION]

Sorry @barnjamin