perf: crypto/types: use native slice index producing for loop in (*CompactBitArry).NumTrueBitsBefore

[odeke-em]

Noticed while examining a bunch of profiles, that the for loop inside (*CompactBitArry).NumTrueBitsBefore unnecessarily consumed a bunch of time:

     7.55s      9.88s (flat, cum) 93.38% of Total
     240ms      250ms     88:func (bA *CompactBitArray) NumTrueBitsBefore(index int) int {
         .          .     89:	onesCount := 0
      70ms      340ms     90:	max := bA.Count()
      70ms       70ms     91:	if index > max {
         .          .     92:		index = max
         .          .     93:	}
         .          .     94:	// below we iterate over the bytes then over bits (in low endian) and count bits set to 1
     2.54s      2.76s     95:	for elem := 0; elem < len(bA.Elems); elem++ {

but we can use the native for loop that produces indices while iterating over slices. Just by simply changing the form results in an improvement

     7.50s      9.95s (flat, cum) 94.94% of Total
     240ms      320ms     88:func (bA *CompactBitArray) NumTrueBitsBefore(index int) int {
         .          .     89:	onesCount := 0
     170ms      420ms     90:	max := bA.Count()
      90ms      100ms     91:	if index > max {
         .          .     92:		index = max
         .          .     93:	}
         .          .     94:	// below we iterate over the bytes then over bits (in low endian) and count bits set to 1
     1.49s      1.62s     95:	for elem := range bA.Elems {

and an improvement in CPU time

$ benchstat before.txt after.txt
name                     old time/op    new time/op    delta
NumTrueBitsBefore/new-8    13.3ns ± 1%    12.5ns ± 1%  -6.07%  (p=0.000 n=10+10)

name                     old alloc/op   new alloc/op   delta
NumTrueBitsBefore/new-8     0.00B          0.00B         ~     (all equal)

name                     old allocs/op  new allocs/op  delta
NumTrueBitsBefore/new-8      0.00           0.00         ~     (all equal)

Fixes #13999