also, previously `Numeric::Nanosecond` had a special left-aligned
parsing behavior. this commit replaces that with a newly designated
`Fixed::Nanosecond` which also handles an empty string which is
possible with an integral number of seconds.
this is most importantly required for negative years in `Parsed`,
which the current parser doesn't generate but is nevertheless
possible in principle. also updates tests for new fields.
this new module encompasses John Nagle's original RFC 2822 and 3337
parsers, updated to fully compatible to the actual standard.
the contributed `parse` module has been merged into it.
Kang Seonghoon