benchstat: accept io.Writer in FormatHTML, not *bytes.Buffer

[nvb]

This brings the API in-line with that of FormatText and FormatCSV and allows it to be used without necessitating an intermediate buffer. The change is fully backwards compatible and doesn't impose any potential performance cost because html.Template.Execute accepts an io.Writer, so any bytes.Buffer passed by reference to FormatHTML was already being forced to escape onto the heap.

This might have been done this way to avoid questions about how the function should handle errors from Writer, but we already ignore those questions and panic in FormatCSV, so it seems reasonable that we would do the same thing here.

[gopherbot]

This PR (HEAD: c5a59e87efa6fb7a1ba8f45110def5e5cec8f6c8) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/perf/+/211320 to see it.

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