Implement augmented assignment operators

Add support for +=, -=, *=, and /= operators. This is basically just
syntactic sugar, but it's still nice to have

    a += 1

compiles to the equivalent of

    a = a + 1

with all the same implications of scoping rules.

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

tools/genast.py

@@ -234,7 +234,7 @@ GENERATE = [
     GenerateGroup("Stmt")
     .add_struct("Import", ["import_kw: Token", "what: Vec<Token>", "module: Token"])
     .add_struct("Expr", ["expr: ExprP"])
-    .add_struct("Assign", ["lhs: Token", "rhs: ExprP"])
+    .add_struct("Assign", ["lhs: Token", "op: Token", "rhs: ExprP"])
     .add_struct("Set", ["expr: ExprP", "name: Token", "rhs: ExprP"])
     .add_struct("Block", ["lbrace: Token", "stmts: Vec<StmtP>", "rbrace: Token"])
     .add_struct("Return", ["return_kw: Token", "expr: Option<ExprP>"])