core/bytes: Tweak index_byte and last_index_byte

- Assume unaligned loads are cheap
- Explicilty use 256-bit or 128-bit SIMD to avoid AVX512
- Limit "vectorized" scanning to 128-bits if SIMD is emulated via SWAR
- Add a few more benchmark cases

tests/benchmark/bytes/benchmark_bytes.odin

@@ -13,9 +13,12 @@ RUNS_PER_SIZE :: 2500
 sizes := [?]int {
 	15, 16, 17,
 	31, 32, 33,
+	63, 64, 65,
+	128,
 	256,
 	512,
 	1024,
+	4096,
 	1024 * 1024,
 	// 1024 * 1024 * 1024,
 }