Add derive based AST visitor (#765)

* Add derive based AST visitor

* Fix BigDecimal

* Fix no visitor feature

* Add test

* Rename visit_table to visit_relation

* Review feedback

* Add pre and post visit

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

derive/README.md

@@ -0,0 +1,79 @@
+# SQL Parser Derive Macro
+
+## Visit
+
+This crate contains a procedural macro that can automatically derive implementations of the `Visit` trait
+
+```rust
+#[derive(Visit)]
+struct Foo {
+    boolean: bool,
+    bar: Bar,
+}
+
+#[derive(Visit)]
+enum Bar {
+    A(),
+    B(String, bool),
+    C { named: i32 },
+}
+```
+
+Will generate code akin to
+
+```rust
+impl Visit for Foo {
+    fn visit<V: Visitor>(&self, visitor: &mut V) -> ControlFlow<V::Break> {
+        self.boolean.visit(visitor)?;
+        self.bar.visit(visitor)?;
+        ControlFlow::Continue(())
+    }
+}
+
+impl Visit for Bar {
+    fn visit<V: Visitor>(&self, visitor: &mut V) -> ControlFlow<V::Break> {
+        match self {
+            Self::A() => {}
+            Self::B(_1, _2) => {
+                _1.visit(visitor)?;
+                _2.visit(visitor)?;
+            }
+            Self::C { named } => {
+                named.visit(visitor)?;
+            }
+        }
+        ControlFlow::Continue(())
+    }
+}
+```
+
+Additionally certain types may wish to call a corresponding method on visitor before recursing
+
+```rust
+#[derive(Visit)]
+#[visit(with = "visit_expr")]
+enum Expr {
+    A(),
+    B(String, #[cfg_attr(feature = "visitor", visit(with = "visit_relation"))] ObjectName, bool),
+}
+```
+
+Will generate
+
+```rust
+impl Visit for Bar {
+    fn visit<V: Visitor>(&self, visitor: &mut V) -> ControlFlow<V::Break> {
+        visitor.visit_expr(self)?;
+        match self {
+            Self::A() => {}
+            Self::B(_1, _2, _3) => {
+                _1.visit(visitor)?;
+                visitor.visit_relation(_3)?;
+                _2.visit(visitor)?;
+                _3.visit(visitor)?;
+            }
+        }
+        ControlFlow::Continue(())
+    }
+}
+```