rand: move dist functions to top module and PRNG interface; minor cleanup (#14481)

vlib/rand/buffer/buffer.v

@@ -0,0 +1,10 @@
+// Copyright (c) 2019-2022 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+module buffer
+
+pub struct PRNGBuffer {
+mut:
+	bytes_left int
+	buffer     u64
+}