Add index assignment and augmented assignment

This allows for syntax like `foo['a'] = 1` and more complex assignments
like `foo.bar()[a() + b()] += 1`

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

tools/genast.py

@@ -256,6 +256,9 @@ GENERATE = [
     .add_struct("Import", ["import_kw: Token", "what: Vec<Token>", "module: Token"])
     .add_struct("Expr", ["expr: ExprP"])
     .add_struct("Assign", ["lhs: Token", "op: Token", "rhs: ExprP"])
+    .add_struct(
+        "IndexAssign", ["expr: ExprP", "index: ExprP", "op: Token", "rhs: ExprP"]
+    )
     .add_struct("Set", ["expr: ExprP", "name: Token", "op: Token", "rhs: ExprP"])
     .add_struct("Block", ["lbrace: Token", "stmts: Vec<StmtP>", "rbrace: Token"])
     .add_struct("Return", ["return_kw: Token", "expr: Option<ExprP>"])