sioutil: add Random and MustRandom helper functions

This commit adds two helper functions for generating
random bytes.

As a user of `sio` you may need to generate a random nonce:
```
import "crypto/rand"

nonce := make([]byte, stream.NonceSize())
if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
   // TODO: error handling
}
```

Such code is quite tedious to write / verbose and has two style
issues:
 - It requires some care to ALWAYS use "crypto/rand" as source
   of entropy. Especially, with editor / IDE tooling like goimports
   it is easy to import an insecure (b/c no CSPRNG) entropy source.
 - Often applications cannot / do not implement reasonable error
   handling. Usually, an application cannot recover into a reasonable
   state when it cannot read from the entropy source of the system.
   This situation is kind of similar to running out-of-memory.

With the new helper functions a library (which may prefer to not panic)
can do just:
```
nonce, err := sioutil.Random(stream.NonceSize())
if err != nil {
   // TODO: error handling - usually return to caller.
}
```

An application may want to call:
`nonce := sioutil.MustRandom(stream.NonceSize())` instead.

sioutil/sio.go

@@ -6,6 +6,7 @@
 package sioutil
 
 import (
+	"crypto/rand"
 	"io"
 
 	"golang.org/x/sys/cpu"
@@ -49,3 +50,30 @@ func NativeAES() bool {
 	return cpu.S390X.HasAES && cpu.S390X.HasAESCBC && cpu.S390X.HasAESCTR &&
 		(cpu.S390X.HasAESGCM || cpu.S390X.HasGHASH)
 }
+
+// Random returns n randomly generated bytes if
+// and only if err == nil.
+//
+// Random uses crypto/rand.Reader as cryptographically
+// secure random number generator (CSPRNG).
+func Random(n int) (b []byte, err error) {
+	b = make([]byte, n)
+	if _, err = io.ReadFull(rand.Reader, b); err != nil {
+		return nil, err
+	}
+	return b, nil
+}
+
+// MustRandom returns n randomly generated bytes.
+// It panics if it fails to read n random bytes
+// from its entropy source.
+//
+// MustRandom uses crypto/rand.Reader as cryptographically
+// secure random number generator (CSPRNG).
+func MustRandom(n int) []byte {
+	b, err := Random(n)
+	if err != nil {
+		panic("sioutil: out of entropy: " + err.Error())
+	}
+	return b
+}