bug#46761: 28.0.50; Speed up json.el encoding

[Basil L. Contovounesios]

Dmitry Gutov <dgutov@yandex.ru> writes:

> I'm guessing there is an approximate size where structures smaller than that
> size will get slower to encode because of this change (creating a temp buffer
> and switching to it are not entirely free)

BTW, we could avoid creating a new buffer each time, e.g. as
json-encode-string did previously, but that comes with its own set of
problems that I think outweigh the benefits.

-- 
Basil