Add "return" statement

Functions may now be exited by returning a value or no value.

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

src/syn/parser.y

@@ -22,6 +22,7 @@ Body -> Result<Vec<Stmt>>:
 Stmt -> Result<Stmt>:
         Expr { Ok(Stmt::Expr($1?)) }
     |   Assign { Ok(Stmt::Assign($1?)) }
+    |   Return { Ok(Stmt::Return($1?)) }
     ;
 
 Assign -> Result<AssignStmt>:
@@ -41,6 +42,11 @@ LhsExpr -> Result<LhsExpr>:
         }
     ;
 
+Return -> Result<ReturnStmt>:
+        'return' Expr { Ok(ReturnStmt { expr: Some($2?), }) }
+    |   'return'      { Ok(ReturnStmt { expr: None, }) }
+    ;
+
 Expr -> Result<Expr>: BinExpr { $1 };
 
 BinExpr -> Result<Expr>: