Change `: ToString` to `: Into<String>`.

[fee1-dead]

The rationale is that if the user already has a String, using ToString will unnecessarily clone the String instead of assigning it directly.

If you want, this can be done without a breaking change (i.e. keeping existing methods and adding new ones to support setting a string directly), but I'd argue that adds too much complexity to the API.

[ISSOtm]

Unfortunately, doing so would lose the ability to use any type that e.g. implements Display. I'm not sure that's great?

[fee1-dead]

Display should be explicit because it might be expensive.