Encode base64 through fixed-size chunks, as clippy 1.99 asks

The encoder takes twelve-byte groups through as_chunks like the decoder
now does, and reads each group's two words with one fixed eight-byte
copy each, which compiles to a load: spelling the bytes out one by one
pushed a 1 MiB save past the 20M fuel budget, while this keeps it at
12.9M.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/codec.rs

@@ -162,6 +162,15 @@ fn quad(word: u64) -> u32 {
     | u32::from(HALVES[1][((word >> 8) & 0xff0f) as usize]) << 16
 }
 
+/// The eight bytes of `group` from `at`, as one little-endian word. Copied as
+/// a fixed eight bytes, the read compiles to a single load; spelling out the
+/// bytes one by one measured several times dearer in fuel.
+fn word(group: &[u8; 12], at: usize) -> u64 {
+  let mut bytes = [0; 8];
+  bytes.copy_from_slice(&group[at..at + 8]);
+  u64::from_le_bytes(bytes)
+}
+
 /// How many characters `bytes` bytes encode to, padding included.
 pub fn encoded_length(bytes: usize) -> usize {
   bytes.div_ceil(3) * 4
@@ -171,23 +180,18 @@ pub fn encoded_length(bytes: usize) -> usize {
 /// exactly `encoded_length` characters. Twelve bytes go through each step
 /// of the main loop, for the same reason sixteen characters do in `decode`.
 pub fn encode(bytes: &[u8], out: &mut [u8]) {
-  let wide = bytes.chunks_exact(12);
-  let (wide_out, rest_out) = out.split_at_mut(bytes.len() / 12 * 16);
-  let rest = wide.remainder();
-  for (group, chars) in wide.zip(wide_out.chunks_exact_mut(16)) {
-    let (Some(low), Some(high)) =
-      (group.first_chunk::<8>(), group.last_chunk::<8>())
-    else {
-      continue;
-    };
-    let low = u64::from_le_bytes(*low);
-    let high = u64::from_le_bytes(*high);
+  let (wide, rest) = bytes.as_chunks::<12>();
+  let (wide_out, rest_out) = out.split_at_mut(wide.len() * 16);
+  let (wide_out, _) = wide_out.as_chunks_mut::<16>();
+  let (rest_out, _) = rest_out.as_chunks_mut::<4>();
+  for (group, chars) in wide.iter().zip(wide_out) {
+    let (low, high) = (word(group, 0), word(group, 4));
     let front = u64::from(quad(low)) | u64::from(quad(low >> 24)) << 32;
     let back = u64::from(quad(high >> 16)) | u64::from(quad(high >> 40)) << 32;
     chars[..8].copy_from_slice(&front.to_le_bytes());
     chars[8..].copy_from_slice(&back.to_le_bytes());
   }
-  for (group, chars) in rest.chunks(3).zip(rest_out.chunks_exact_mut(4)) {
+  for (group, chars) in rest.chunks(3).zip(rest_out) {
     let mut padded = [0; 8];
     padded[..group.len()].copy_from_slice(group);
     chars.copy_from_slice(&quad(u64::from_le_bytes(padded)).to_le_bytes());