cmd/bent: add version tags; benchmark curation and repair

Could be split into two CLs (.go files and .toml files)
but for testing this is good enough.

Change-Id: I2c1ee06a4917c5edc22c1e009a855dc4d4436061
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/354634
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>