This commit adds an `ImplicitSelfKind` to the HIR and the MIR that keeps
track of whether a implicit self argument is immutable by-value, mutable
by-value, immutable reference or mutable reference so that the addition
of the `mut` keyword can be suggested for the immutable by-value case.
DefaultImpl is a highly confusing name for what we now call auto impls,
as in `impl Send for ..`. The name auto impl is not formally decided
but for sanity anything is better than `DefaultImpl` which refers
neither to `default impl` nor to `impl Default`.
Instead of collecting all potential inputs to some metadata entry and
hashing those, we directly hash the values we are storing in metadata.
This is more accurate and doesn't suffer from quadratic blow-up when
many entries have the same dependencies.
