add slice operation functions

[gammazero]

The following functions have been added to operate on slices of values:

• AppendToSlice(out []T) []T
• CopyInSlice([]T)
• CopyOutSlice([]T) int

Are these worth adding to Deque?

AppendToSlice

out = q.AppendToSlice(out) 

is an efficient shortcut for

for i := 0; i < q.Len(); i++ {
     x = append(out, q.At(i))
}

CopyInSlice

q.CopyInSlice(in)

is an efficient shortcut for

q.Clear()
for i := range in { 
    q.PushBack(in[i])
}

CopyOutSlice

n := q.CopyOutSlice(out)

is an efficient shortcut for

n := min(len(out), q.Len()) 
for i := 0; i < n; i++ {
    out[i] = q.At(i)
}

Copy

n := b.Copy(a)

is an efficient shortcut for

b.Clear()
n := a.Len()
b.Grow(n))
for i := 0; i < n; i++ {
    b.PushBack(a.At(i))
}

Replaces #37