encoding/json: avoid some more pointer receivers

A few encoder struct types, such as map and slice, only encapsulate
other prepared encoder funcs. Using pointer receivers has no advantage,
and makes calling these methods slightly more expensive.

Not a huge performance win, but certainly an easy one. The struct types
used in the benchmark below contain one slice field and one pointer
field.

name           old time/op    new time/op    delta
CodeEncoder-4    5.48ms ± 0%    5.39ms ± 0%  -1.66%  (p=0.010 n=6+4)

name           old speed      new speed      delta
CodeEncoder-4   354MB/s ± 0%   360MB/s ± 0%  +1.69%  (p=0.010 n=6+4)

Updates #5683.

Change-Id: I9f78dbe07fcc6fbf19a6d96c22f5d6970db9eca4
Reviewed-on: https://go-review.googlesource.com/131400


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>