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 issue for further info
Projections can generate lifetime variables with equality constraints,
that will not be resolved by `resolve_type_vars_if_possible`, so substs
need to be lifetime-erased after that.
Fixes#36848.
Instead of finding aux-build files in `auxiliary`, we now search for an
`aux` directory relative to the test. So if your test is
`compile-fail/foo.rs`, we would look in `compile-fail/aux`. Similarly,
we ignore the `aux` directory when searching for tets.
