truncating timestamps on arbitrary intervals

Hi,

When analyzing time-series data, it's useful to be able to bin
timestamps into equally spaced ranges. date_trunc() is only able to
bin on a specified whole unit. In the attached patch for the March
commitfest, I propose a new function date_trunc_interval(), which can
truncate to arbitrary intervals, e.g.:

select date_trunc_interval('15 minutes', timestamp '2020-02-16
20:48:40'); date_trunc_interval
---------------------
 2020-02-16 20:45:00
(1 row)

With this addition, it might be possible to turn the existing
date_trunc() functions into wrappers. I haven't done that here because
it didn't seem practical at this point. For one, the existing
functions have special treatment for weeks, centuries, and millennia.

Note: I've only written the implementation for the type timestamp
without timezone. Adding timezone support would be pretty simple, but
I wanted to get feedback on the basic idea first before making it
complete. I've also written tests and very basic documentation.

-- 
John Naylor                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services