Support annotations for methods in impl blocks

[coeff-aij]

In the current implementation every function associated with a struct is treated as a top‑level function.
Consequently, annotations on methods must use the low‑level form is_double(*self, ^self) instead of the natural method syntax.

Example

• Before

impl A {
    #[thrust::requires(true)]
    #[thrust::ensures(is_double(*self, ^self))]   // must write the receiver manually
    fn double(&mut self) { … }
}

• After

impl A {
    #[thrust::requires(true)]
    #[thrust::ensures(*self.is_double(^self))]    // natural method‑call syntax
    fn double(&mut self) { … }
}

[coeff-aij]

This feature seems to be orthogonal to support for annotations on traits, so its implementation can be delayed.