Avoids std::distance calls

[François Dumont]

On 04/06/2018 23:09, Jonathan Wakely wrote:
> On 04/06/18 22:13 +0200, François Dumont wrote:
>> Hi
>>
>> I'd like to propose this patch to avoid std::distance calls. In a 
>> number of situation in algos we already have the size of the buffer 
>> we need so we shouldn't have to compute it again.
>
> Just one place, in __inplace_merge, no?

For the moment yes but I'll submit another patch soon.

Now committed.