[v3] delay _M_fill initialization

[Paolo Carlini]

Benjamin Kosnik wrote:

>This delays the initialization of _M_fill untill the first call of
>basic_ios::fill(). By doing this, things like basic_istream<unsigned
>short> can use unformatted I/O without adding non-required facets to
>the imbued locale.
>
My personal, humble opinion is that this is very very nice! Good work 
Benjamin!

P.