Historically, we struggled with formulating the right grammar for block expressions. Today's EffectExpr is the best we've come up so far, but, if you are thinking "WTF is an effect expression?", you are not wrong. I think in this commit I've come up with what can be called a reasonable grammar for block expressions. Observe that *all* things in `{}` we call list: item list, assoc item list, match arm list, record field list, record expr field list. In fact, `BlockExpr` is the only exception. So, let's just call the stuff in `{}` a statement list. This isn't forced: *all* things inside a block are statements, and `;` is a statement separator, just like `,`. Trailing `;` is allowed, but not required. Then, statement list with modifiers such as `async` or attributes or labels is just a block expression. Why haven't I thought of it from the start?