Shrink To Fit trustline

[JeremyRubin]

This proposes a new operation which is equivalent of a shrink_to_fit for trustlines.

The core purpose of this operation is to avoid a situation where attempts at closing a trustline fail because someone sends a small amount of asset to a trustline which is not at the limit, preventing a change trust from decreasing it.

a shrink to fit operation should set the trust line's trust limit to the smallest size to accommodate liabilities and balances.

This should be easily backwards compatible as the behavior is an emulable interleaving of existing transactions.