Add keepIf proc and keepIfIt template to sequtils

It's faster to filter sequences inplace, as can be seen with this:
```nimrod
import sequtils

var x: seq[int] = @[]
for i in 1..100_000_000:
  x.add(i)

#var y = filter(x, proc(a): bool = a mod 2 == 0) # 1.8 seconds
#echo y[1000]
#echo y.len

keepIf(x, proc(a): bool = a mod 2 == 0) # 0.6 seconds
echo x[1000]
echo x.len
```

I'm not sure about the name. We can't use filter (unlike with map's two procs) because the inplace filter has the same arguments. filterDirectly is too long, removeIf (as in C++) would require negating the condition proc, which is why I chose keepIf.