encoding/json: revert "avoid work when unquoting strings, take 2"

This reverts golang.org/cl/190659 and golang.org/cl/226218, minus the
regression tests in the latter.

The original work happened in golang.org/cl/151157, which was reverted
in golang.org/cl/190909 due to a crash found by fuzzing.

We tried a second time in golang.org/cl/190659, which shipped with Go
1.14. A bug was found, where strings would be mangled in certain edge
cases. The fix for that was golang.org/cl/226218, which was backported
into Go 1.14.4.

Unfortunately, a second regression was just reported in #39555, which is
a similar case of strings getting mangled when decoding under certain
conditions. It would be possible to come up with another small patch to
fix that edge case, but instead, let's just revert the entire
optimization, as it has proved to do more harm than good. Moreover, it's
hard to argue or prove that there will be no more such regressions.

However, all the work wasn't for nothing. First, we learned that the way
the decoder unquotes tokenized strings isn't simple; initially, we had
wrongly assumed that each string was unquoted exactly once and in order.
Second, we have gained a number of regression tests which will be useful
to prevent the same mistakes in the future, including the test cases we
add in this CL.

Fixes #39555.

Change-Id: I66a6919c2dd6d9789232482ba6cf3814eaa70f61
Reviewed-on: https://go-review.googlesource.com/c/go/+/237838
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

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