adding a clean function and a prealloc buffer support

[vitock]

in some case , using prealloced memory would be a better choice

it would be appreciated that if you offer a clean function to reset the limbs to zero .
keeping temp sensitive data on memory is not a good choice

maybe codes below say more

func afunction(b:inout Bint){
    var temporaryObjects = BInt alloc from Buffer Pool user provide
    .
    .
    .
    
    /// do the CLEAN job, 
    temporaryBints.SetLimbsBytesToZero()

    temporaryBints set all the storage data to 0;
}