cmd/trace: factor out durationHistogram

This code will be useful for the new tracer, and there's no need to
duplicate it. This change copies it to internal/trace/traceviewer, adds
some comments, and renames it to TimeHistogram.

While we're here, let's get rid of the unused String method which has a
comment talking about how awful the rendering is.

Also, let's get rid of uses of niceDuration. We'd have to bring it
with us in the move and I don't think it's worth it. The difference
between the default time.Duration rendering and the niceDuration
rendering is usually a few extra digits of precision. Yes, it's noisier,
but AFAICT it's not substantially worse. It doesn't seem worth the new
API, even if it's just internal. We can also always bring it back later.

For #60773.
For #63960.

Change-Id: I795f58f579f1d503c540c3a40bed12e52bce38e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/542001
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

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