Bancor strategy

[matthewh86]

Attempt at the Bancor Strategy bounty.

I don't think I did too terribly considering I didn't know Solidity on Friday 😅

areas accounted for (or at least code exists for it)

• 100 day withdraw lock (to accrue Liquidity Protection against IL)
• withdraw lock configured at construction to allow for easier testing
• trading BNT for the collateral token
• keeping track of each liquidity position
• partial withdraws of liquidity
• interest tracking
• override withdraw that will ignore liquidity protection lock in
• fee transfers
• I aimed to make base BancorStrategy token agnostic, so hopefully should be extendible to LINK etc.

tech debt

• I couldn't get the Bancor integration tests to work, as the behaviour to simulate market conditions did not seem to alter the results I was getting back for the removeLiquidityReturn call.
• I didn't get time to work out what some of the tests were attempting (e.g. the compound one obtained the exchange rate several times)
• some troubleshooting / debug code left in
• lots of tests aren't passing due to lack of time
• I did not initially realise that the BNT compensation had a 24H lock before being released to the account. spare BNT should still be hoovered up whenever a withdrawal is done though