Merge #41: Implement missing indexops for Arith expressions

c757014 Implement missing indexops for Arith expressions (sanket1729)
9d230c8 Refactor to use txEnv in arith (sanket1729)

Pull request description:

ACKs for top commit:
  apoelstra:
    ACK c757014

Tree-SHA512: f59613720db10af5ef909ae80a2de18e1b0c62cffdfaa864e3485de3940703e5d5afa033c17be580dfebbe0ccfd85c616a7bb3e19ad7a8732a7e1c9d430cb435

Author: sanket1729