Small strings (up to 3 bytes on 32-bit, up to 7 bytes on 64-bin) can be represented
as 'immediates' of struct Lisp_String, without allocating data separately. I observed
two usage scenarios (editing with a few tens of buffers and long byte-force-recompile
runs), and in both cases ~10% of live strings are less than 4 bytes long, and ~30% of
live strings are less than 8 bytes long. So I think it's worth playing with such a
little complication.

Dmitry