Allocator Vault Support

[hexonaut]

Contains:

• ALM Delegate Controller
• Linear Fee Swap Pool
• Gated Swap Pool
• Offchain Gated Swap Pool

Integration Tests:

• USDC Linear Fee Swap Pool
• GUSD Linear Fee Swap Pool
• USDP Linear Fee Swap Pool
• Backed Finance IB01 Linear Fee Swap Pool
• Arrakis (GUNI) USDC/DAI 1bps Gated Swap Pool
• Offchain Monetalis / Block Tower

Some changes to the general interface:

IPool has liquidityAvailable() to return the total amount of liquidity that is immediately available in the pool. IPool has idleLiquidity() to determine how much dai should be discounted from fee calculations. This is not used now, but will be required later when fees are fully automated. IPlan getTargetAssets() takes in the ilk now to allow a single contract for multiple ilks. IFees added to allow custom accounting when fees are returned instead of assuming everything goes to the vow. For now everything is set to just send everything to the vow.

[hexonaut]

I'm going to add some more tests, but this is getting close to being finished for review.

[lucas-manuel]

Blocked by: https://github.com/makerdao/dss-direct-deposit/pull/111