Implement -[NSData writeToURL:options:error:]

[lowell]

Cleaned up resubmission of #57, powered by awesome feedback™

[phausler]

Building on linux i get this failure

Foundation/NSData.swift:444:45: error: cannot convert value of type 'NSURL?' to expected argument type 'Bool'
        guard let path = url.path where url.filePathURL == true else {
                                        ~~~~^~~~~~~~~~~

which is really strange since i don't get that on mac

[parkera]

This is almost ready, but there is one thing to resolve here. The as NSString cast fails on Linux without the ObjC bridging enabled:

Foundation/NSData.swift:445:57: error: cannot convert value of type 'String' to type 'NSString' in coercion
            let userInfo = [NSLocalizedDescriptionKey : "The folder at “\(url)” does not exist or is not a file URL." as NSString, // NSLocalizedString() not yet available

we could add the explicit bridge cast here, but actually I think that NSError's userInfo should be [String : Any] instead of [String : AnyObject]. This is consistent with the changes we've been making elsewhere so we can return Swift.String instead of Foundation.NSString, since the lack of bridging otherwise makes it difficult to use by the caller.

[parkera]

I pushed some other fixes for NSError so that we could split that problem from this one.