Expand ZeroFrom to cover cases where attrs can borrow

[Manishearth]

This adds zerofrom(may_borrow), which allows deriving ZeroFrom on generic types which have parameters we care about borrowing.

I'm not happy about the terminology in ZF, we have a couple things talking about the same thing:

• the "C type", which is the type fed into zero_from, and the type parameter of ZeroFrom
• 'zf_inner, which is the lifetime of the "C type"
• lifetime_ty, which is the C type version of the field type. usually the type with 'zf_inner replacing its main lifetime

I'd like to come up with a nice name for these things we can make consistent. I try to document this a bit better here though.

I'm doing this in an attempt to start using generics in datetime symbols code more; I have #3767 open but I want to try and clean up the rest.

If this approach works I plan to do something similar in Yoke.

may_borrow can't be applied directly to type parameters due to rust-lang/rust#114393. This code is future-proof for whenever that upstream bug is fixed.

Part of #3771

[sffc]

Seems about right. It's been a little while since I've looked at this derive code so it's possible there's things I missed.