Implement Display for HashParts instead of ToString directly

[ZakDggn]

The documentation for ToString states that

ToString shouldn’t be implemented directly: Display should be implemented instead, and you get the ToString implementation for free.

The ToString implementation also used the old default version (2y). HashParts::format() can be called instead which uses the new default version (2b). This required changing HashParts::format() to take a reference to self instead of consuming it.

[Keats]

Looks like a legit test failure?

[ZakDggn]

Oops! I hadn't noticed that one of the tests was using to_string(). I've updated it to use 2b instead of 2y and all the tests are passing now.