Add a relaxed datetime parser. This datetime parser functions by using `chrono`s
own parsing utilities and a try/succeed approach to parsing.

This implementation of the datetime parser has some drawbacks and some
positives. On the positive side:
- it was easy to implement
- it is easy to add more datetime formats to

In order to add additionally supported formats, a developer can add the
required format string to the `format` mod in `parse_datetime.rs`, and
then add it as a potential format to the relevant `fmts` vec.

On the negative:
- It is not easily customiseable beyond the supported `chrono` parsing
  formats. E.g., `chrono` does not currently support parsing offsets
  without trailing zeros. `from_str("UTC+1")` should return a valid response
  but `chrono` fails to parse this.
- Because it is an attempt driven parser, it is likely not that
  performant. I have not done any performance testing as part of this
change, but I would expect a custom parser to perform much better.

Co-authored-by: Sylvestre Ledru <sledru@mozilla.com>

Update the README and add a test module to parse_datetime for any
examples presented in the README.

add datetime parser

Also fuzz fuzz_parse_datetime_from_str for a minute

rename from humantime_to_duration to parse_datetime