Replace for loop with [u8]::fill method

[joshlf]

In map_zeroed and map_prefix_tuple_zeroed, we zero a byte slice by doing:

for b in lv.0.iter_mut() {
    *b = 0;
}

Instead, we could use [u8]::fill:

lv.0.fill(0);

[MSalah73]

I'll take this one

[joshlf]

Awesome, thanks @MSalah73! Welcome to zerocopy 😃