Mapping implementation

[Shawn Steele]

FWIW: There were a few mails discussing mapping order and the like.

Internally, for the current IDNA2003 version, instead of handling each IDNA2003 step, like running NFKC, then applying the additional IDN mappings, etc., we instead have do the mapping in a single pass.  Basically our APIs think that "IDNA" is a special form of Normalization and we just add an extra table for it.  Of course for the Bidi rules and extra label rules we have to do some extra checking, but the mapping step is done all at once.  I'd expect that other implementations handle it similarly.  (Which is why indirectly part of why some implementations give you the 2. mappings on the whole name instead of the labels, etc.)

Though this doesn't directly impact the mapping document, it might be worth keeping in mind that implementers will likely consolidate the steps to avoid multiple passes.

-Shawn