enable setter(into) at struct level

[evbo]

per #82 could we make this one step easier like derive-builder does by allowing setter(into) for the entire struct? Then, so long as Into is implemented for any field it saves you from having to type setter(into) for each one individually.

Not a big deal, but... ya know.. heated seats...

[Tockra]

Would be nice. I'm currently want to provide a builder pattern in a file with 98 structs which mostly has Option<> as fields, but I have not the motivation to add the builder(default, setter(strip_option)) to each field (multiple thousand lines).

[Tockra]

I used the power of AI and created a PR:
#159

Maybe further changes are needed ...

[ifiokjr]

This can be closed as field_default already exists.