Fix kext warning

Use fixed-length vectors for AES tables

Note that because we have `default Order dec`, Sail vectors
are indexed the same as bitvectors, in decreasing order, hence why
the indexing in sbox_lookup becomes
```
table[255 - unsigned(x)]
```
rather than
```
table[unsigned(x)]
```
The alternative would be to flip all the literals

model/riscv_types_kext.sail

@@ -163,7 +163,7 @@ function aes_decode_rcon(r) = {
 }
 
 /* SM4 SBox - only one sbox for forwards and inverse */
-let sm4_sbox_table : list(bits(8)) = [|
+let sm4_sbox_table : vector(256, bits(8)) = [
 0xD6, 0x90, 0xE9, 0xFE, 0xCC, 0xE1, 0x3D, 0xB7, 0x16, 0xB6, 0x14, 0xC2, 0x28,
 0xFB, 0x2C, 0x05, 0x2B, 0x67, 0x9A, 0x76, 0x2A, 0xBE, 0x04, 0xC3, 0xAA, 0x44,
 0x13, 0x26, 0x49, 0x86, 0x06, 0x99, 0x9C, 0x42, 0x50, 0xF4, 0x91, 0xEF, 0x98,
@@ -184,9 +184,9 @@ let sm4_sbox_table : list(bits(8)) = [|
 0xE5, 0xB4, 0xB0, 0x89, 0x69, 0x97, 0x4A, 0x0C, 0x96, 0x77, 0x7E, 0x65, 0xB9,
 0xF1, 0x09, 0xC5, 0x6E, 0xC6, 0x84, 0x18, 0xF0, 0x7D, 0xEC, 0x3A, 0xDC, 0x4D,
 0x20, 0x79, 0xEE, 0x5F, 0x3E, 0xD7, 0xCB, 0x39, 0x48
-|]
+]
 
-let aes_sbox_fwd_table : list(bits(8)) = [|
+let aes_sbox_fwd_table : vector(256, bits(8)) = [
 0x63, 0x7c, 0x77, 0x7b, 0xf2, 0x6b, 0x6f, 0xc5, 0x30, 0x01, 0x67, 0x2b, 0xfe,
 0xd7, 0xab, 0x76, 0xca, 0x82, 0xc9, 0x7d, 0xfa, 0x59, 0x47, 0xf0, 0xad, 0xd4,
 0xa2, 0xaf, 0x9c, 0xa4, 0x72, 0xc0, 0xb7, 0xfd, 0x93, 0x26, 0x36, 0x3f, 0xf7,
@@ -207,9 +207,9 @@ let aes_sbox_fwd_table : list(bits(8)) = [|
 0xc1, 0x1d, 0x9e, 0xe1, 0xf8, 0x98, 0x11, 0x69, 0xd9, 0x8e, 0x94, 0x9b, 0x1e,
 0x87, 0xe9, 0xce, 0x55, 0x28, 0xdf, 0x8c, 0xa1, 0x89, 0x0d, 0xbf, 0xe6, 0x42,
 0x68, 0x41, 0x99, 0x2d, 0x0f, 0xb0, 0x54, 0xbb, 0x16
-|]
+]
 
-let aes_sbox_inv_table : list(bits(8)) = [|
+let aes_sbox_inv_table : vector(256, bits(8)) = [
 0x52, 0x09, 0x6a, 0xd5, 0x30, 0x36, 0xa5, 0x38, 0xbf, 0x40, 0xa3, 0x9e, 0x81,
 0xf3, 0xd7, 0xfb, 0x7c, 0xe3, 0x39, 0x82, 0x9b, 0x2f, 0xff, 0x87, 0x34, 0x8e,
 0x43, 0x44, 0xc4, 0xde, 0xe9, 0xcb, 0x54, 0x7b, 0x94, 0x32, 0xa6, 0xc2, 0x23,
@@ -230,17 +230,15 @@ let aes_sbox_inv_table : list(bits(8)) = [|
 0xc9, 0x9c, 0xef, 0xa0, 0xe0, 0x3b, 0x4d, 0xae, 0x2a, 0xf5, 0xb0, 0xc8, 0xeb,
 0xbb, 0x3c, 0x83, 0x53, 0x99, 0x61, 0x17, 0x2b, 0x04, 0x7e, 0xba, 0x77, 0xd6,
 0x26, 0xe1, 0x69, 0x14, 0x63, 0x55, 0x21, 0x0c, 0x7d
-|]
+]
 
-/* Lookup function - takes an index and a list, and retrieves the
- * x'th element of that list.
+/* Lookup function - takes an index and a table, and retrieves the
+ * x'th element of that table. Note that the Sail vector literals
+ * start at index 255, and go down to 0.
  */
-val sbox_lookup : (bits(8), list(bits(8))) -> bits(8)
+val sbox_lookup : (bits(8), vector(256, bits(8))) -> bits(8)
 function sbox_lookup(x, table) = {
-  match (x, table) {
-    (0x00, t0::tn) => t0,
-    (   y, t0::tn) => sbox_lookup(x - 0x01, tn)
-  }
+  table[255 - unsigned(x)]
 }
 
 /* Easy function to perform a forward AES SBox operation on 1 byte. */