bug#42147: 28.0.50; pure vs side-effect-free, missing optimizations?

[Mattias Engdegård]

>> But yes, it's a problem because `C-h o kbd RET` says explicitly that it
>> doesn't change the match data :-(
> 
> Motivation enough for them to save the match data then.

I pushed the save-match-data around 'kbd' since that one looked rather obvious.

Regarding package-get-version: perhaps we should drop the 'pure' property and 
just let callers wrap it in eval-when-compile?