impl Timestamp::{Display, FromStr} with RFC-3339 format

This PR implements to and from string methods for
`prost_types::Timestamp` following the RFC-3339 timestamp format, as
also specified by the [Protobuf JSON mapping spec][1]. The `Display`
impl always emits the Zulu (Z) timezone, whereas the parser is slightly
lenient in accepting fixed timezone offsets, as well as both the `T` and
space seperated date and time portions. This leniency in parsing makes
it much much more robust to parsing data of less-than-pristine
formatting, even though the protobuf spec allows no leniency.

Also, since it was easy with the new date time functionality, we've gone
ahead and added a trio of convenience construct functions to `Timestamp`
which makes it straightforward to create `Timestamp`s out of calendar
date time information.

This functionality will be used in follow-up PRs to add Serde
[de]serialization support to the types in `prost_types`, which is itself
a stepping stone towards full JSON support.

This PR was extracted from tokio-rs#558, and is based on code which myself and
@konradjniemiec have written, and which has been running in production
at @sisudata for a few years. The to-string portion of this code has
also been incorporated into [`tracing`][2], so it should be pretty well
battle hardened.

[1]: https://developers.google.com/protocol-buffers/docs/proto3#json
[2]: https://github.com/tokio-rs/tracing/blob/master/tracing-subscriber/src/fmt/time/datetime.rs