Add the actual chain of projections to `UserTypeProjection`.

Update the existing NLL `patterns.rs` test accordingly.

includes changes addressing review feedback:

 * Added example to docs for `UserTypeProjections` illustrating how we
   build up multiple projections when descending into a pattern with
   type ascriptions.

 * Adapted niko's suggested docs for `UserTypeProjection`.

 * Factored out `projection_ty` from more general `projection_ty_core`
   (as a drive-by, made its callback an `FnMut`, as I discovered later
   that I need that).

 * Add note to docs that `PlaceTy.field_ty(..)` does not normalize its result.

 * Normalize as we project out `field_ty`.

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