feat: move RpoRandomCoin and define Rng trait

nits: minor

chore: update log and readme

src/rand/mod.rs

@@ -1,3 +1,18 @@
 //! Pseudo-random element generation.
 
 pub use winter_crypto::{RandomCoin, RandomCoinError};
+
+use crate::{Felt, Word, ZERO};
+
+mod rpo;
+
+/// Pseudo-random element generator.
+///
+/// An instance can be used to draw, uniformly at random, basefield elements as well as `Word`s.
+pub trait FeltRng {
+    /// Draw, uniformly at random, a basefield element.
+    fn draw_element(&mut self) -> Felt;
+
+    /// Draw, uniformly at random, a `Word`.
+    fn draw_word(&mut self) -> Word;
+}