cmd/bent, sweet/common: allow running single benchmarks in isolation

This includes a tool for converting the output of a
bent run into a benchmarks.toml file with each benchmark
run in isolation.

This required a few changes to bent so that if more than
one benchmark is run from the same binary, that binary will
only be compiled/initialized once per configuration, and
also changes to escapify "horrible" benchmark names that
would otherwise produce problematic file names (e.g.,
problematic file names for profiles).

Copyfile in bent was modified to allow the copy to succeed
if the destination already exists and contains the expected
content, even if its permissions do not allow writing (which
happens).

Change-Id: I4b0d424685c254a86a099ea4f655b689cdcf009a
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/807720
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>