Add reversed proc

A more efficient reverse if you need a new seq instead of working inplace. Only traverses the input array once and writes the output seq once. Offers better performance and is shorter to write:
```nimrod
var c = @[0]
for i in 1..10000000:
  c.add(i)

# 2.6 s
for i in 0..10:
  var d = c
  reverse(d)

# 1.5 s
for i in 0..10:
  var d = reversed(c)
```