Add static_assert attribute

```
 @graydon | if someone wants a cute task
 @graydon | I would be fond of an attribute called #[static_assert]
 @graydon | that, when applied to a static bool, halts compilation if the bool evaluates to false.
 @graydon | and is an error (or ignored) anywhere else
 @graydon | maybe optionally #[static_assert("why this is important")]
```
