Linter: Warn when number primitive annotated with `#[ink(topic)]`

[cmichi]

The #[ink(topic)] annotation enables third party tooling to filter for "topics" ‒ discrete events for which it makes sense to filter. Typical examples where this can make sense are AccountId, bool or enum variants.

It typically doesn't make sense to annotate types like u32 or i32 as a topic, if those fields can take continuous values that could be anywhere between ::MIN and ::MAX. An example of a case where it doesn't make sense at all to have a topic on the storage field is something like value: Balance.

We should provide a linter rule that warns developers when they annotate a number primitive with #[ink(topic)].

[cmichi]

I'm putting some notes here on how to approach this issue:

The linting stage in ink! is optional by now. You have to install the linting dependencies and can then execute cargo contract build --lint on a contract. With the ink! 4.0 release we deprecated the only one lint we had. So at the moment there won't be any lint run.

A lint was introduced in this PR. The files to get you started are in ink_linting/src/. Those two are most relevant:

• ink_linting/src/mapping_initialized.rs: This is the linting rule.
• ink_linting/src/lib.rs: This is where the linting rule is registered, so that it's actually executed.

There are some more interesting things in this PR (tests), but the above should serve as a starting point.