Add BoundedSeparated with parser and printer

This type should be able to replace Separated (with a bit of effort).

The hope is that this type is a better representation for table-like
syntax with optional trailing delimiter that will let me remove_map
elements without too much difficulty. This is necessary for desugaring
table constructors.

src/ast/basic.rs

@@ -96,3 +96,16 @@ impl<E, S1, S2> HasSpan for Separated<E, S1, S2> {
         }
     }
 }
+
+#[derive(Debug, Clone)]
+pub struct BoundedSeparated<E> {
+    pub elems: Vec<(Space, E, Space)>,
+    pub trailing: Option<Space>,
+    pub span: Span,
+}
+
+impl<E> HasSpan for BoundedSeparated<E> {
+    fn span(&self) -> Span {
+        self.span
+    }
+}