add poll_map feature

[robjtede]

checks off issue 63514 from #16

[jplatte]

Thanks for the PR :)

Can you split this into two files? I've generally been doing that for feature flags that are for a handful of methods on one type (as opposed to sth. like pointer or integer methods that are named the same thing but available on a bunch of distinct types, where I use items).

[jplatte]

One more thing: the titles should be enclosed in backticks (so they get rendered as <code>).