lending: cross collateral / cross liquidity support

[jordaaash]

This PR adds support for multiple collateral deposits and liquidity borrows per obligation (currently set to 10 total). Obligation tokens are retained, specific to each collateral and minted 1:1.

Backwards compatibility was not a design goal. Obligation, reserve, and lending market structs, methods, serialized objects, and errors have been significantly changed.

Serum dex prices are no longer used. solana-flux-aggregator is used instead. We'll need to provide oracles for reserve currencies to the lending market quote currency.

Issues

• Closes lending: Support cross collateralized loans #993: many-to-many collateral / liquidity is supported; obligation tokens are removed; fees now come out of liquidity
• Closes lending: Reduce liquidation payment for defaulted loans #1138: liquidations now adjust repay amount based on the value of available collateral
• Closes lending: Stabilize price discovery #992, closes lending: No validation for dex market account in reserve initialization #996: using dex market impact price for refreshing market values presents various issues; using a TWAP/VWAP/oracle could be more efficient and accurate; for now an oracle is used
• Closes lending: Support borrow / lending between two non-quote reserves #989: all lending market values are now in quote currency (provided oracle support)
• Closes lending: Add support for borrowing with native tokens as collateral #998: it's possible to deposit, refresh an obligation, and borrow in the same transaction
• Affects lending: Add slippage constraint for borrow instruction #988: borrows are now either an exact amount or 100% of available borrowing power; we could apply a minimum amount of liquidity to receive in the latter case; potentially also applies to repay and liquidate instructions

[jstarry]

This is looking great, time to get this in and start iterating with smaller incremental PR's