[FEATURE REQUEST] Trim some fat off Printf by removing rarely used features

[glennsl]

A few months ago on the OCaml forums @gasche mentioned:

I did a small experiment removing %{ fmt %}, %( fmt %) and conversion between formats and runtime strings, and I get a 40% size reduction (from 56Kio to 36Kio) of minified js. (Edit: in fact 16Kio, so a 70% size reduction.)

Just figured I'd bring this up here too, so it's not forgotten.