Wrapping a forward range in another forward range

[Rudy Raab]

So I have an XLSX (MS Excel 2007+ file format) library that I 
wrote  (https://github.com/TransientResponse/dlang-xlsx) that I 
recently converted from std.xml to dxml. That went well and it 
still works (much faster too).

Now I had the idea that a lazy forward range API (like dxml 
itself or std.csv) would be more convenient than my current API, 
which reads an XLSX sheet in its entirety as a giant array of 
arrays (string[][]).

So I set about creating a wrapper class that implements front 
(returns a string[], the current row), popFront (runs through the 
dxml range until it reaches the end of the next row and populates 
the current row array from that), and empty.

I've figured out that my wrapper Sheet class has to be a 
templated class (don't know why) in order for my empty method to 
not fail on matching std.range.primitive's definition of empty.

Since I'm trying to create a range of string[]s, I templated my 
class as
```
class XLSheet(R) if(isRandomAccessRange!R && 
isSomeChar!(ElementType!R))
```

And now when I go to use it, if I declare it as say:
```
auto sheetRange = new XLSheet!(string[])();
```

DMD (2.080.1 Win10 x64) gives me the error:
```
source\xlsx.d(203,27): Error: template instance 
`xlsx.XLSheet!(string[])` does not match template declaration 
XLSheet(R) if (isRandomAccessRange!R && 
isSomeChar!(ElementType!R))
```

I'm guessing that `string[]` doesn't satisfy the requirements of 
`isRandomAccessRange` and such.

Can anyone smarter than me (or at least better at D than me) 
educate me as to why this doesn't compile, and how I might go 
about defining my wrapper class?

Full source code is on GitHub as I mentioned, in the `rangeAPI` 
branch.