This PR is meant as an alternative to #902 and #614, for the problem in described in #575.

The problem is that if the format string is invalid, or contains a field that the type can not provide (such as time fields on a NaiveDate), the display implementation returns an error.

Some formattings macro's can correctly forward this error, such as write! and writeln!. Others, such as println!, format! and to_string panic instead.

• fix format panic problem #614 proposed to ignore any fields that return an error when formatting
• Make StrftimeItems::new return Result #902 proposed to make StrftimeItems::new return a Result. It will need some more work to detect a string with fields the type can not provide.
• This PR proposes to add methods that return fmt::Result<String>.

Compared to #902 I believe this solution is better because:

• It works on 0.4.x, no breaking changes
• It makes it easier to get a String, instead of making it slightly harder to get a DelayedFormat.
• There is no need to parse the formatting string twice.

There are 5 new methods, all just a couple of lines of code:

• DelayedFormat::try_to_string
• NaiveDate::format_to_string
• NaiveTime::format_to_string
• NaiveDateTime::format_to_string
• DateTime::format_to_string

Most of the effort in this PR is updating the surrounding documentation and examples, to make users more aware of the potential panic and the solutions.