The `localtime` crate was defined by cloudhead, and is a minimal
repository with a single `lib.rs`.
Instead of using it as an external dependency, copy the code directly
into a new workspace crate `radicle-localtime`.
The default `serde` implementation goes through the `LocalTime`'s
seconds values rather than milliseconds, since this is the more common
format. This allows the removal of the `serde_ext` functions.
The one place milliseconds was used was for the
`radicle::cob::common::Timestamp` type, where the `Serialize` and
`Deserialize` implementations are manually written.
It also adds a `schemars` feature to remove `schemars_ext` functions
in `radicle` as well.
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