Implement write_len for FormattedDateTime

[sffc]

write_len can improve the performance of to-string operations. This issue is to track implementation of write_len for FormattedDateTime.

[sffc]

I'm no longer convinced that we need or want this. It would likely involve walking the whole formatting code and pattern string, which may or may not outweigh the benefit of having the hint. We may want to consider a way to return an approximate hint instead of an exact hint from this function.