NodeIds are sequential integers starting at zero, so we can achieve some
memory savings by just storing the items all in a line in a vector.
The occupancy for typical crates seems to be 75-80%, so we're already
more efficient than a HashMap (maximum occupancy 75%), not even counting
the extra book-keeping that HashMap does.
Dead code pass now explicitly checks for `#[allow(dead_code)]` and
`#[lang=".."]` attributes on items and marks them as live if they have
those attributes. The former is done so that if we want to suppress
warnings for a group of dead functions, we only have to annotate the
"root" of the call chain.
