Merge 'Cursor cleanups' from Pekka Enberg

Clean up the cursor lookup code in VDBE in preparation for multi-version
cursor support.

Reviewed-by: Jussi Saurio <jussi.saurio@gmail.com>

Closes #1090

core/types.rs

@@ -619,20 +619,15 @@ impl Record {
 }
 
 pub enum Cursor {
-    Table(BTreeCursor),
-    Index(BTreeCursor),
+    BTree(BTreeCursor),
     Pseudo(PseudoCursor),
     Sorter(Sorter),
     Virtual(VTabOpaqueCursor),
 }
 
 impl Cursor {
-    pub fn new_table(cursor: BTreeCursor) -> Self {
-        Self::Table(cursor)
-    }
-
-    pub fn new_index(cursor: BTreeCursor) -> Self {
-        Self::Index(cursor)
+    pub fn new_btree(cursor: BTreeCursor) -> Self {
+        Self::BTree(cursor)
     }
 
     pub fn new_pseudo(cursor: PseudoCursor) -> Self {
@@ -643,17 +638,10 @@ impl Cursor {
         Self::Sorter(cursor)
     }
 
-    pub fn as_table_mut(&mut self) -> &mut BTreeCursor {
+    pub fn as_btree_mut(&mut self) -> &mut BTreeCursor {
         match self {
-            Self::Table(cursor) => cursor,
-            _ => panic!("Cursor is not a table"),
-        }
-    }
-
-    pub fn as_index_mut(&mut self) -> &mut BTreeCursor {
-        match self {
-            Self::Index(cursor) => cursor,
-            _ => panic!("Cursor is not an index"),
+            Self::BTree(cursor) => cursor,
+            _ => panic!("Cursor is not a btree"),
         }
     }