Auto merge of #37860 - giannicic:defaultimpl, r=nagisa

Auto merge of #37860 - giannicic:defaultimpl, r=nagisa

#37653 support `default impl` for specialization

this commit implements the first step of the `default impl` feature:

> all items in a `default impl` are (implicitly) `default` and hence
> specializable.

In order to test this feature I've copied all the tests provided for the
`default` method implementation (in run-pass/specialization and
compile-fail/specialization directories) and moved the `default` keyword
from the item to the impl.
See [referenced](https://github.com/rust-lang/rust/issues/37653) issue for further info

r? @aturon