0.138.0: Reconcile margin after risk-reducing order fills

[MathisWellmann]

Changes:

• version 0.138.0
• Fixed previous panic asserting available_balance >= 0, which was violated after a risk-reducing market order got filled and margin was no longer sufficient.
• New method reconcile_margin ensures account collateral is reconciled after a fill was settled.
• liquidate refactored into force_liquidate, avoiding possible panic paths by not going through public interface with additional checks.
• New types ForcedCancels, Solvency and MarketOrderSettlement to ensure forced cancels of limit orders are propagated nicely through the public API.
• Bunch more new methods on Account to help with everything.
• Account gained a bad_debt field of losses absorbed by the venue (insurance fund / auto-deleveraging style)
• Now position reducing orders are always accepted by the venue (as before), but the panicking assumptions are eliminated.