all: add more uses of stringer

By grepping for ]string{$, one can find many manual implementations of
stringer. The debug/dwarf ones needed the new -trimprefix flag, too.

html/template was fairly simple, just implementing the fallback as
stringer would. The changes there are trivial.

The ones in debug/dwarf needed a bit of extra logic since the GoString
wants to use its own format, depending on whether or not the value is
one of the known constants.

Change-Id: I501ea7deaa538fa425c8e9c2bb895f480169273f
Reviewed-on: https://go-review.googlesource.com/77253


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>