render functions: render in a buffer

[gasche]

Rendering does not use any pretty-printing commands, so it does not
need the expressivity of Format. Using a Buffer directly is just as
flexible (we can output to a format or a string later), yet simpler
and faster. In a rendering microbenchmark I see a 20% speedup.

[gasche]

Note: it would be easier to reduce the small code duplication in the With_locations version if the interface was:

val render_fmt : Formatter.t -> ?strict -> ?partials -> t -> Json.t -> unit
val render_buf : Buffer.t    -> ?strict -> ?partials -> t -> Json.t -> unit
val render :                    ?strict -> ?partials -> t -> Json.t -> string

instead of

val render_fmt : ?strict -> ?partials -> Formatter.t -> t -> Json.t -> unit
val render_buf : ?strict -> ?partials -> Buffer.t    -> t -> Json.t -> unit
val render :     ?strict -> ?partials                -> t -> Json.t -> string

I considered this change in the PR, but in theory it could break some users, so I refrained from it.