UtcOffsetCalculator should have a borrowed variant

[sffc]

I found this because it's confusing that ZonedDateTime::try_from_str takes one borrowed thing and one thing by value:

pub fn try_from_str(
    ixdtf_str: &str,
    calendar: A,
    iana_parser: IanaParserBorrowed<'_>,
    offset_calculator: &UtcOffsetCalculator,
)

CC @robertbastian

[sffc]

• @robertbastian What was the litmus test?
• @sffc If the operation is fairly lightweight relative to the operation
• @Manishearth We care about destructors, too
• @sffc Another thing is it allows us to have an optional alloc feature
• @robertbastian It seems like this doesn't really pass the litmus test so it should be a stretch
• @Manishearth I think it's fine if someone asks for it

Conclusion: add this issue to 2.0-stretch