[v3] Mt_alloc: use __bin._M_free[0]

[Paolo Carlini]

Nathan Myers wrote:

>I see no reason to remove "explicit".  It is good practice to put it on 
>all constructors except where you have deliberately decided to make an 
>implicit conversion.  Somebody might add a (perhaps defaulted) argument, 
>or (on a two-argument constructor) change one of them to defaulted, or 
>just remove it, and forget to finish the job.  As it is, it's harmless,
>but under minor changes it protects against introducing a bug.
>
>It's too bad that the language has the wrong default.  The original
>proposal (that the core WG replaced with "explicit") suggested a new 
>default syntax that would have been more natural and convenient.
>  
>
Thanks for the interesting explanation, Nate!

Ok, I'm not going to touch those two 'explicit'.

"Ubi major"... ;)

Paolo.