std: pass bytes.Buffer and strings.Builder by pointer
This CL fixes a number of (all true positive) findings of vet's
copylock analyzer patched to treat the Bu{ff,uild}er types
as non-copyable after first use.
This does require imposing an additional indirection
between noder.writer and Encoder since the field is
embedded by value but its constructor now returns a pointer.
Updates golang/go#25907
Updates golang/go#47276
Change-Id: I0b4d77ac12bcecadf06a91709e695365da10766c
Reviewed-on: https://go-review.googlesource.com/c/go/+/635339
Reviewed-by: Robert Findley <rfindley@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>