Improvements to the strategy implemented by `{fixed,standard,zip317}::SingleOutputChangeStrategy`

[daira]

zcash_client_backend::{fixed,standard,zip317}::SingleOutputChangeStrategy now implement a different strategy for choosing whether there will be any change, and its value. The aims are:

• Ensure that it is still possible to create fully transparent transactions with no change (this will be needed for ZIP 320). The ChangeError::InsufficientFunds error in this case should have a required field that reflects the additional amount needed, according to the fee calculated without an extra change output.
• Avoid leaking information about note amounts in some cases: an adversary that knew the number of external recipients and the sum of their outputs was able to learn the sum of the inputs if no change output was present.
• Defend against losing money by using DustAction::AddDustToFee with a too-high dust threshold.
• Ensure that if a "change memo" is requested, there will always be a shielded change output in which to put it. Previously, this would not be the case when using DustAction::AddDustToFee.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 21 lines in your changes missing coverage. Please review.

Project coverage is 63.16%. Comparing base (06c0895) to head (7710919).

:exclamation: Current head 7710919 differs from pull request most recent head 21d5731

Please upload reports for the commit 21d5731 to get more accurate results.

Files	Patch %	Lines
zcash_client_backend/src/fees/common.rs	75.00%	21 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1430 +/- ## ========================================== + Coverage 63.08% 63.16% +0.08% ========================================== Files 127 127 Lines 14821 14868 +47 ========================================== + Hits 9350 9392 +42 - Misses 5471 5476 +5 ```

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