Improve diagnostics of missing #[pin_data] and #[pin]

[nbdd0121]

Improve the diagnostics of missing #[pin_data] and #[pin], by adding
diagnostics attributes. For #[pin_data], we have to change the expansion
to retrieve PinData in a two-step process, so we change the error message
from "method exists but trait bound not satisfied" to "trait not
implemented".

This is the new error message for missing #[pin_data]:

error[E0277]: `Foo` cannot be used with `pin_init!` macro
 --> tests/ui/compile-fail/init/missing_pin_data.rs:9:19
  |
9 |         pin_init!(Self { a: 42 })
  |                   ^^^^ unsatisfied trait bound
  |
help: the trait `pin_init::__internal::HasPinData` is not implemented for `Foo`

where it was previously

error[E0599]: no associated function or constant named `__pin_data` found for struct `Foo` in the current scope
 --> tests/ui/compile-fail/init/missing_pin_data.rs:9:9
  |
3 | struct Foo {
  | ---------- associated function or constant `__pin_data` not found for this struct
...
9 |         pin_init!(Self { a: 42 })
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^ associated function or constant not found in `Foo`
  |
  = help: items from traits can only be used if the trait is implemented and in scope
  = note: the following trait defines an item `__pin_data`, perhaps you need to implement it:
          candidate #1: `pin_init::__internal::HasPinData`

and this is the new error message for missing #[pin]:

error[E0277]: `impl PinInit<usize>` cannot be used to movably initialize `usize` with error `_`
  --> tests/ui/compile-fail/pin_data/missing_pin.rs:12:18
   |
12 |             a <- a,
   |             -----^
   |             |    |
   |             |    the trait `Init<usize, _>` is not implemented for `impl PinInit<usize>`
   |             required by a bound introduced by this call
   |
   = note: if your type implements `PinInit` but not `Init`, you might be forgetting a `#[pin]` annotation on fields

where it was previously

error[E0277]: the trait bound `impl pin_init::PinInit<Bar>: Init<Bar, _>` is not satisfied
  --> tests/ui/compile-fail/init/invalid_init.rs:19:16
   |
19 |         bar <- Bar::new(),
   |         -------^^^^^^^^^^
   |         |      |
   |         |      the trait `Init<Bar, _>` is not implemented for `impl pin_init::PinInit<Bar>`
   |         required by a bound introduced by this call