crypto: implement rand.bytes(needed_bytes int) ?[]byte, use it consistently instead of the old rand.read(), which will change to be compatible with io and the pseudo random rand module

vlib/crypto/README.md

@@ -21,7 +21,7 @@ import crypto.rand
 
 fn main() {
 	// remember to save this key somewhere if you ever want to decrypt your data
-	key := rand.read(32) ?
+	key := rand.bytes(32) ?
 	println('KEY: $key')
 
 	// this data is one block (16 bytes) big