add gocritic/ruleguard checks back again, cleanup code. (#13665)

- remove some duplicated code
- reported a bug, separately fixed in #13664
- using strings.ReplaceAll() when needed
- using filepath.ToSlash() use when needed
- remove all non-Go style comments from the codebase

Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>

cmd/streaming-signature-v4.go

@@ -436,7 +436,7 @@ func parseHexUint(v []byte) (n uint64, err error) {
 	for i, b := range v {
 		switch {
 		case '0' <= b && b <= '9':
-			b = b - '0'
+			b -= '0'
 		case 'a' <= b && b <= 'f':
 			b = b - 'a' + 10
 		case 'A' <= b && b <= 'F':