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

vlib/crypto/bcrypt/bcrypt.v

@@ -59,7 +59,7 @@ pub fn compare_hash_and_password(password []byte, hashed_password []byte) ? {
 
 // generate_salt generate a string to be treated as a salt.
 pub fn generate_salt() string {
-	randbytes := rand.read(bcrypt.salt_length) or { panic(err) }
+	randbytes := rand.bytes(bcrypt.salt_length) or { panic(err) }
 	return randbytes.bytestr()
 }
 

vlib/crypto/rand/rand.v

@@ -11,3 +11,13 @@ struct ReadError {
 pub fn (err ReadError) msg() string {
 	return 'crypto.rand.read() error reading random bytes'
 }
+
+// bytes returns an array of `bytes_needed` random bytes.
+// NB: this call can block your program for a long period of time,
+// if your system does not have access to enough entropy.
+// See also rand.bytes(), if you do not need really random bytes,
+// but instead pseudo random ones, from a pseudo random generator
+// that can be seeded, and that is usually faster.
+pub fn bytes(bytes_needed int) ?[]byte {
+	return read(bytes_needed)
+}