Adding unixtime property to Timestamp and DatetimeIndex
This PR implements #43975 (https://github.com/pandas-dev/pandas/issues/43975), adding a unixtime property to Timestamp and Datetime index.

In the course of developing this PR, I discovered that the Timestamp class already has a method called timestamp() which calculates a unixtime. I deprecated the timestamp method, as I found the name ambiguous (there's nothing about the phrase timestamp that leads me to think of unixtimes). 

This is my first PR against pandas, so apologies for any violations of conventions for code or PRs.
Hello @fergalm! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:









There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2022-01-31 00:44:13 UTC This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. closing as stale. we would still take this if rebased & updated. pls ping to reopen if interested.