UTF16 support

[vtjnash]

I would like to merge UTF16 support into base. My reason is that all the good windows API's require UTF16 support, so it would be nice to have native support for this third ByteString String type. (see also #1919)

My intention was to move the code from the UTF16 package to Base:
https://github.com/nolta/UTF16.jl (@nolta)

Please let me know your concerns so I can attempt to address them.

[JeffBezanson]

UTF16 is not a ByteString.

[vtjnash]

In what way?

[JeffBezanson]

In that nearly every method we have defined for ByteString will not work on UTF16. ByteString has something to do with the basic storage unit being a byte, but really I think the only meaning one can ascribe to it is "ASCII or UTF8". Originally the only purpose of it was so we could have an ASCII vs. UTF8 distinction for performance, but not think about the difference most of the time. At this stage I would prefer folding all three of those types into one.

[StefanKarpinski]

I.e. ASCIIString, UTF8String and ByteString. The only other potential candidate for ByteStringiness is Latin1 strings, but I'm not even sure that's a good idea. We've gotten remarkably far without ever needing to pass a Latin1 string to any C API.

[vtjnash]

OK. It doesn't sound like there are any objections then? Just don't call it a bytestring? I'm quite all right with that.

[StefanKarpinski]

Yes, it's clearly something we need to do for Windows.

[nolta]

Fine by me. I'll move the code over.

[StefanKarpinski]

We should probably leave the UTF16 as a do-nothing stub that prints a warning when loaded since people may well still be loading it for a while.

[StefanKarpinski]

^ package

[nolta]

I was thinking about setting UTF16's REQUIRE to julia 0.2- 0.3?

[StefanKarpinski]

Sorry, huge reopen button on a small phone. So that 0.3 is a hard stop on when people can even have 0.3 installed through the package manager?

[vtjnash]

Wow, that was fast. Thanks all! I may try to do some work on cleaning up the windows API stuff over thanksgiving (unless someone else wants to take it -- i would be more than happy to give it away).

[nolta]

@StefanKarpinski Not sure i understand your comment. Would julia 0.2- 0.3- be better?

[StefanKarpinski]

If it's just a stub, I'm not clear on why capping that allowable Julia version matters.

[stevengj]

To convert binary data to UTF-16, see also #6546.