Add augmented assignment operators for set statements

This allows us to do things like

    self.foo += 5

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/parser.rs

@@ -587,13 +587,24 @@ impl Parser {
             let expr = self.expr()?;
             let stmt: StmtP;
 
-            if expr.as_any_ref().downcast_ref::<GetExpr>().is_some() && self.mat(TokenKind::Eq)? {
+            if expr.as_any_ref().downcast_ref::<GetExpr>().is_some()
+                && mat!(
+                    self,
+                    TokenKind::Eq,
+                    TokenKind::PlusEq,
+                    TokenKind::MinusEq,
+                    TokenKind::StarEq,
+                    TokenKind::SlashEq
+                )
+            {
+                let op = self.prev.clone().unwrap();
                 let expr = expr.as_any().downcast::<GetExpr>().unwrap();
                 let rhs = self.expr()?;
                 // unpack the GetExpr and turn it into a SetExpr instead
                 stmt = Box::new(SetStmt {
                     expr: expr.expr,
                     name: expr.name,
+                    op,
                     rhs,
                 });
             } else {