Merge 'Add drop index' from Anton Harniakou

This commit adds suport for DROP INDEX.
Bytecode produced by this commit differs from SQLITE's bytecode, main
reason we don't do autovacuum or repacking of pages like SQLITE does.
Closes #1280

Closes #1444

core/vdbe/explain.rs

@@ -1187,6 +1187,15 @@ pub fn insn_to_str(
                 0,
                 format!("DROP TABLE {}", table_name),
             ),
+            Insn::DropIndex { db: _, index } => (
+                "DropIndex",
+                0,
+                0,
+                0,
+                OwnedValue::build_text(index.name.clone()),
+                0,
+                format!("DROP INDEX {}", index.name),
+            ),
             Insn::Close { cursor_id } => (
                 "Close",
                 *cursor_id as i32,