fmt: add serialization and deserialization for UnsignedDuration (std::time::Duration)

[aalhendi]

This PR begins to address #298.

src/fmt/friendly/printer.rs

• Introduces jiff::fmt::friendly::SpanPrinter::print_unsigned_duration method which accepts as core::timeDuration as UnsignedDuration, converts to a SignedDuration and reuses the existing print_duration method.

src/fmt/serde.rs

• Add module jiff::fmt::serde::unsigned_duration to house serde helpers for core::time::Duration
  • jiff::fmt::serde::unsigned_duration::friendly::compact holds required and optional serialization functions. Similar to jiff::fmt::serde::duration::friendly::compact
  • jiff::fmt::serde::unsigned_duration::deserialize holds required and optional deserializers.