Replace proplists with maps for timings and sizes

[tsloughter]

Is there any objection to this? Maps have been available for the last 3 major releases, going on 4 major releases soon enough. Proplists don't make much sense for things that don't need the true/false functionality.

[tsloughter]

Bleh, maybe not worth it, would have to replace the lists:filtermap with a foldl that builds the map... hm

[yurrriq]

I don't object. We don't officially support OTP versions <18 anyway.