feat: `#[pavex::methods]` must return an error if applied to an `impl` block that doesn't contain `#[pavex::*]`-annotated methods

[LukeMathWalker]

#[pavex::methods] exists to give other macros (e.g. #[pavex::get]) the required context when they are applied to a method.

There is no reason to apply #[pavex::methods] to an impl block that doesn't contain annotated methods. Such an occurrence may be a symptom of the user not having fully internalized the way Pavex's machinery works: we should emit an error with a proper explanation to nudge them in the right direction.

Example

This code snippet should be rejected at compile-time:

pub struct A;

#[pavex::methods]
impl A {
    pub fn new() -> Self {
        A
    }
}

[Duckaet]

@LukeMathWalker , can you look into the error message, is it pointing in right direction or should i clarify it more?