internal/export/idna: finalize API

Made the API configuration internal.
This is a little bit safer but also gives some flexibilty to
add features that may require computed setup and/or
tables that one should not link in by default.
Examples of such possible features are confusable and
spoofing detection.

Updates golang/go#17268

Change-Id: Ia205d17feb385a4e9fea47287ca32786f80da0e1
Reviewed-on: https://go-review.googlesource.com/32155
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed