Adding UniswapV2 properties

[tuturu-tech]

Initial draft of UniswapV2 basic properties.

Providing liquidity

• always increases K
• always increases LP token supply
• always increases user LP token supply
• never impacts token prices
• always increases token reserves

Withdrawing liquidity

• always decreases K
• always decreases LP token supply
• always decreases user LP token supply
• never impacts token prices
• always decreases token reserves

Swapping

• never decreases K
• swapping in both directions does not result in tokens gained
• always increases user token out balance
• price of tokenIn always decreases
• price of tokenOut always increases

Blockers: Properties fail with ErrorUnrecognizedOpcode https://github.com/crytic/echidna/issues/984, so far haven't been able to figure out why Cons: Only tests UniswapV2 pair and router. Next step will be to generalize this so the properties can be easily used in a harness for testing UniV2-based AMMs