AMM: Atomic deposit and add liquidity script

[supiket]

Motivation

Currently, adding liquidity to an exchange contract requires three steps:

1. Deposit some amount of asset A using deposit()
2. Deposit some amount of asset B using deposit()
3. Add liquidity using add_liquidity()

A Sway script can do this atomically.

Specification

The script should accept asset IDs and the amounts to be deposited to add liquidity by using up at least one of the assets (this depends on the ratio of the values of the two assets). Should refund any extra amount.