strings: use Builder in Map

[rsc]

@robpike and I were just looking at strings.Map. It should use Builder to avoid a copy at the end in the conversion from []byte to string.

[gopherbot]

Change https://golang.org/cl/122875 mentions this issue: strings: use Builder in Map

[fraenkel]

There is a performance issue that I ran across when trying to use the Builder. The last loop which copies by byte or rune should be able to to just use builder.WriteRune(r), however, it was always ~30% slower to do so.

[zigo101]

Builder is not always faster, in particular for this specified case: https://groups.google.com/forum/#!topic/golang-nuts/I6Ch7hMEQLk

[josharian]

@fraenkel want to file an independent issue?

[fraenkel]

@josharian We did determine its due to inlining. There is also another issue around which is focused on strings vs []bytes, #26264

[iamoryanmoshe]

Is this issue still relevant? Should I check it out?

[gopherbot]

Change https://golang.org/cl/137575 mentions this issue: strings: use Builder in ToUpper and ToLower