Support UTF-32 position encoding

Looks like this is a native encoding for Emacs at least!

crates/stdx/src/rand.rs

@@ -0,0 +1,21 @@
+//! We don't use `rand`, as that's too many things for us.
+//!
+//! Currently, we use oorandom instead, but it misses these two utilities.
+//! Perhaps we should switch to `fastrand`, or our own small prng, it's not like
+//! we need anything move complicatied that xor-shift.
+
+pub fn shuffle<T>(slice: &mut [T], mut rand_index: impl FnMut(usize) -> usize) {
+    let mut remaining = slice.len() - 1;
+    while remaining > 0 {
+        let index = rand_index(remaining);
+        slice.swap(remaining, index);
+        remaining -= 1;
+    }
+}
+
+pub fn seed() -> u64 {
+    use std::collections::hash_map::RandomState;
+    use std::hash::{BuildHasher, Hasher};
+
+    RandomState::new().build_hasher().finish()
+}