List of permitting structures in const functions is out of date

The list [here](https://doc.rust-lang.org/nightly/reference/items/functions.html#const-functions) of things permitted in const functions is out of date, and has been since at least [1.33.0](https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1330-2019-02-28) when `let` bindings, assignments, and other constructs were allowed.