rand: extend PRNG interface, add buffering support (#13608)

vlib/rand/rand.js.v

@@ -66,9 +66,8 @@ pub fn ulid_at_millisecond(unix_time_milli u64) string {
 	return res
 }
 
-// read fills in `buf` a maximum of `buf.len` random bytes
-pub fn read(mut buf []byte) {
+fn read_internal(mut rng PRNG, mut buf []byte) {
 	for i in 0 .. buf.len {
-		buf[i] = byte(default_rng.u32())
+		buf[i] = rng.byte()
 	}
 }