Vendor dependencies

Let's see how I like this workflow.

vendor/chrono/README.md

@@ -0,0 +1,59 @@
+[Chrono][docsrs]: Date and Time for Rust
+========================================
+
+[![Chrono GitHub Actions][gh-image]][gh-checks]
+[![Chrono on crates.io][cratesio-image]][cratesio]
+[![Chrono on docs.rs][docsrs-image]][docsrs]
+[![Join the chat at https://gitter.im/chrono-rs/chrono][gitter-image]][gitter]
+
+[gh-image]: https://github.com/chronotope/chrono/actions/workflows/test.yml/badge.svg
+[gh-checks]: https://github.com/chronotope/chrono/actions?query=workflow%3Atest
+[cratesio-image]: https://img.shields.io/crates/v/chrono.svg
+[cratesio]: https://crates.io/crates/chrono
+[docsrs-image]: https://docs.rs/chrono/badge.svg
+[docsrs]: https://docs.rs/chrono
+[gitter-image]: https://badges.gitter.im/chrono-rs/chrono.svg
+[gitter]: https://gitter.im/chrono-rs/chrono
+
+It aims to be a feature-complete superset of
+the [time](https://github.com/rust-lang-deprecated/time) library.
+In particular,
+
+* Chrono strictly adheres to ISO 8601.
+* Chrono is timezone-aware by default, with separate timezone-naive types.
+* Chrono is space-optimal and (while not being the primary goal) reasonably efficient.
+
+There were several previous attempts to bring a good date and time library to Rust,
+which Chrono builds upon and should acknowledge:
+
+* [Initial research on
+   the wiki](https://github.com/rust-lang/rust-wiki-backup/blob/master/Lib-datetime.md)
+* Dietrich Epp's [datetime-rs](https://github.com/depp/datetime-rs)
+* Luis de Bethencourt's [rust-datetime](https://github.com/luisbg/rust-datetime)
+
+## Limitations
+
+Only proleptic Gregorian calendar (i.e. extended to support older dates) is supported.
+Be very careful if you really have to deal with pre-20C dates, they can be in Julian or others.
+
+Date types are limited in about +/- 262,000 years from the common epoch.
+Time types are limited in the nanosecond accuracy.
+
+[Leap seconds are supported in the representation but
+Chrono doesn't try to make use of them](https://docs.rs/chrono/0.4/chrono/naive/struct.NaiveTime.html#leap-second-handling).
+(The main reason is that leap seconds are not really predictable.)
+Almost *every* operation over the possible leap seconds will ignore them.
+Consider using `NaiveDateTime` with the implicit TAI (International Atomic Time) scale
+if you want.
+
+Chrono inherently does not support an inaccurate or partial date and time representation.
+Any operation that can be ambiguous will return `None` in such cases.
+For example, "a month later" of 2014-01-30 is not well-defined
+and consequently `Utc.ymd_opt(2014, 1, 30).unwrap().with_month(2)` returns `None`.
+
+Non ISO week handling is not yet supported.
+For now you can use the [chrono_ext](https://crates.io/crates/chrono_ext)
+crate ([sources](https://github.com/bcourtine/chrono-ext/)).
+
+Advanced time zone handling is not yet supported.
+For now you can try the [Chrono-tz](https://github.com/chronotope/chrono-tz/) crate instead.