Better support for BLOBs

- Limbo command line shell supports e.g. `SELECT x'616263';`
- `EXPLAIN SELECT x'616263';` lists the opcode

Missing:

- Command line shell not entirely compatible with SQLite when blobs have
  non-printable characters in the middle (e.g. `SELECT x'610062';`)
- Python bindings not supported (incoming soon)

testing/select.test

@@ -11,6 +11,18 @@ do_execsql_test select-const-2 {
   SELECT 2
 } {2}
 
+do_execsql_test select-blob-empty {
+  SELECT x'';
+} {}
+
+do_execsql_test select-blob-ascii {
+  SELECT x'6C696D626f';
+} {limbo}
+
+do_execsql_test select-blob-emoji {
+  SELECT x'F09FA680';
+} {🦀}
+
 do_execsql_test select-limit-0 {
   SELECT id FROM users LIMIT 0;
 } {}