Where to put new Scheme engravers?

[Urs Liska]

Hi devs,

sorry for the uninformed question, but I don't seem to be able to find 
an answer in the CG or in the source directory.

If I have a new Scheme engraver that I'd like to suggest to be added, 
where would I look for a place to add it to?
Is there a file (or a set of files grouped by topic) where I should 
insert it or can I add a dedicated file (in which folder)?

TIA
Urs