Serialization data corruption with Duration and `serde_human_readable` for negative subsecond durations

[vriesk]

The issue is here:

time/time/src/serde/mod.rs

Line 257 in aca5c36

self.whole_seconds(),

When feature serde_human_readable is enabled, the serializing code incorrectly assumes that for a sub-second negative duration, the self.whole_seconds() will provide the sign during printing. Similar issue exists with deserialization code.

The bug will result with subsecond negative Durations being serialized into positive ones, and negative elsewhere-serialized Durations to be deserialized into positive ones.

Also the comment here:

time/time/src/duration.rs

Line 44 in aca5c36

/// Number of nanoseconds within the second. The sign always matches the `seconds` field.

is incorrect - the nanoseconds sign will NOT match the seconds sign when seconds is 0 and nanoseconds is negative.

[vriesk]

Suggested fix here: #692