The ToASCII operation takes a sequence of Unicode code points that
make up one domain label and transforms it into a sequence of code
points in the ASCII range (0..7F). If ToASCII succeeds, the
original sequence and the resulting sequence are equivalent labels.
It is important to note that the ToASCII operation can fail. ToASCII
fails if any step of it fails. If any step of the ToASCII operation
fails on any label in a domain name, that domain name MUST NOT be used
as an internationalized domain name. The method for deadling with this
failure is application-specific.
The inputs to ToASCII are a sequence of code points, the AllowUnassigned
flag, and the UseSTD3ASCIIRules flag. The output of ToASCII is either a
sequence of ASCII code points or a failure condition.
ToASCII never alters a sequence of code points that are all in the ASCII
range to begin with (although it could fail). Applying the ToASCII
operation multiple times has exactly the same effect as applying it just
Return value: Returns 0 on success, or an Idna_rc error code.