Implement N3762 string_view: a non-owning reference to a string.

[Jonathan Wakely]

On 20 November 2013 14:15, Jonathan Wakely wrote:
> On 20 November 2013 14:09, Ed Smith-Rowland wrote:
>>
>> Here is a fix that introduces an constexpr unit length string to represent
>> empty strings so that refs to empty strings don't point to the ether.
>
> Does that static const need an out-of-class definition?

Oh sorry, I hadn't got as far as string_view.tcc!