std.array.Appender needs a way to extend/add to the length of `data`.  See the following use case:

```
uint tryAppendFullPathNameImpl(const(wchar)* nullTerminatedPath, Appender!(wchar[]) builder)
{
    import core.sys.windows.winbase : GetFullPathNameW;
    auto prefixLength = builder.data.length;
    for (;;)
    {
        const result = GetFullPathNameW(nullTerminatedPath, builder.capacity - prefixLength,
            builder.data.ptr + prefixLength, null);
        if (result <= (builder.capacity - prefixLength))
        {
            // NO WAY TO DO THIS:
            //builder.overrideDataLength(prefixLength + result);
            return result;
        }
        builder.reserve(prefixLength + result);
    }
}
```

What's happening here is we are passing the Appender array to a C function that populates the array with our resuling "full path".  Note that this implementation is taken from .NET CoreFX:

https://github.com/dotnet/corefx/blob/1bff7880bfa949e8c5e46039808ec412640bbb5e/src/Common/src/CoreLib/System/IO/PathHelper.Windows.cs#L72

The problem is that once it's populated, we have no way of extending the length of the array after it was populated by the C function.