A very useful and widely used time operation is to approximate a given delay or age (in seconds) using a human-readable unit — think "2 hours", "5 days",  "3 weeks", or "7 months".  We have `seconds-to-string', but it provides more precision than is often required, skips some meaningful "human readable" duration units like weeks and months, and uses abbreviated units exclusively.  

For those familiar with magit, the `magit--age' function has provided this capability for quite some time (e.g. for short commit age), and other packages have adapted it.  It would be useful to have a version in core.

This patch provides a `seconds-to-string-approximate' function based loosely on `magit--age' and `seconds-to-string'.  It allows using abbreviated or full units, and can optionally round to the nearest half-unit.