set_unset & friends for ActiveValue

[PacificBird]

PR Info

• Closes "Set if unset" methods for ActiveValues #3077

New Features

• ActiveValue::set_unset(&mut self, value: V), which mutates self to ActiveValue::Set(value) if self.is_not_set()
• ActiveValue::set_unset(&mut self, value: impl FnOnce() -> V), which mutates self to ActiveValue::Set(f()) if self.is_not_set()
• ActiveValue::set_unset(&mut self) where V: Default, which mutates self to ActiveValue::Set(V::default()) if self.is_not_set()

Includes documentation and examples/doc tests for all three. I cannot overstate how much grief these changes will save me in my production code base.

[Huliiiiii]

I'd prefer the names set_if_unset / not_set, even though they’re a bit more verbose.
What do you think? @tyt2y3