also changes the behavior of `Numeric::Nanosecond` (`%f`) to
the left-aligned digits and allows for the wider range of time zone
offsets from -99:59 to +99:59.
- Format string is internally represented as a series of formatting
items. Items can be directly given to now-public `format::format`
function as well.
- Format string parser is separated to `format::strftime` module.
This is to allow for potentional alternative formatting syntaxes.
- `DelayedFormat` now receives an iterator for formatting items.
Kang Seonghoon