Give a better error message for `async pub fn` and `const pub fn`

[pickfire]

It may be possible that developers get mixed up where pub visibility needs to be shown.

error: expected item, found keyword `async`
 --> src/lib.rs:1:1
  |
1 | async pub fn t() {}
  | ^^^^^ expected item

error: aborting due to previous error

error: expected identifier, found keyword `pub`
 --> src/lib.rs:1:7
  |
1 | const pub fn t() {}
  |       ^^^ expected identifier, found keyword

error: expected one of `:`, `;`, or `=`, found keyword `fn`
 --> src/lib.rs:1:11
  |
1 | const pub fn t() {}
  |           ^^ expected one of `:`, `;`, or `=`

error: missing type for `const` item
 --> src/lib.rs:1:7
  |
1 | const pub fn t() {}
  |       ^^^ help: provide a type for the item: `r#pub: <type>`

error: aborting due to 3 previous errors

I am working on the first one, the second one looks harder to solve so I am not solving it.

It would be good to suggest the developer that pub needs to come before async or const. And I don't even know if async should come before const or the other way around.