Fixed the slippage validation logic according to swap market pair

[steven-semnet]

The following changes have been made:

• Created a MarketPair struct in swap.rs file and use it to decide if the swap is selling or buying the base asset.
• Updated validate_slippage logic with market pair
• Updated all the tests for slippage validation.

Reference: tidechain PR-356

Tests Results:

running 16 tests
test assets::tests::test_saturation_of_assets ... ok
test swap::test::tests::slippage_1_000_ath_to_10_usdc ... ok
test swap::test::tests::slippage_1_btc_to_100_tdfy ... ok
test swap::test::tests::slippage_100_tdfy_to_1_btc ... ok
test swap::test::tests::slippage_0_dot_9_btc_to_5_dot_9_eth ... ok
test swap::test::tests::slippage_1_btc_to_30_000_usdc ... ok
test swap::test::tests::slippage_10_usdc_to_1_000_ath ... ok
test swap::test::tests::slippage_14_btc_to_2_900_000_tdfy ... ok
test swap::test::tests::slippage_1_000_tdfy_to_2_900_usdc ... ok
test swap::test::tests::slippage_24_700_tdfy_to_5_eth ... ok
test swap::test::tests::slippage_2_900_usdc_to_1_000_tdfy ... ok
test swap::test::tests::slippage_5_dot_9_eth_to_0_dot_9_btc ... ok
test swap::test::tests::slippage_2_900_usdc_to_5_eth ... ok
test swap::test::tests::slippage_30_000_usdc_to_1_btc ... ok
test swap::test::tests::slippage_5_eth_to_2_900_usdc ... ok
test swap::test::tests::slippage_5_eth_to_24_700_tdfy ... ok

[elliotekj]

While I do feel that this whole module could be made a lot more readable, the logic itself looks good to me and tests derived from the RFC examples are passing. Good job!

[elliotekj]

LGTM

[steven-semnet]

LGTM

Thanks :)