prune: speed up using 1<<N instead of math.Pow(2, N)

This change fixes the use of expensive code that used
math.Pow(float(2), N) but really that's expensive code
that could be simplified by using left bitshifts.

The improvements in speed show in benchmarks:

```shell
$ benchstat before.txt after.txt
name    old time/op    new time/op    delta
Pow2-8     322ns ± 2%     114ns ± 1%  -64.69%  (p=0.000 n=9+8)

name    old alloc/op   new alloc/op   delta
Pow2-8     40.0B ± 0%     40.0B ± 0%     ~     (all equal)

name    old allocs/op  new allocs/op  delta
Pow2-8      5.00 ± 0%      5.00 ± 0%     ~     (all equal)
```

Fixes #72

subrootpaths.go

@@ -71,7 +71,9 @@ func prune(idxStart uint, idxEnd uint, maxWidth uint) [][]int {
 	rightTraversed := false
 
 	for i := treeDepth - 1; i >= 0 && capturedSpan < idxEnd; i-- {
-		nodeSpan := uint(math.Pow(float64(2), float64(treeDepth-i)))
+		// nodeSpan is 2**(treeDepth-i) == 1<<(treeDepth-i)
+		// Please see: https://github.com/celestiaorg/nmt/issues/72
+		nodeSpan := 1 << (treeDepth - i)
 		if pathStart[i] == 0 {
 			// if nodespan is less than end index, continue traversing upwards
 			lastNode := nodeSpan + idxStart - 1