lib/hashcol: forgot to nullify a link in `HashCollection::enlarge`

Thus caused buggy bucklet-link update in `HashSet::remove`
Thus caused buggy transitive reduction in `POSet::add_edge`
Thus caused buggy coloration in `layout_builders`
Thus caused buggy type test tables in `SeparateCompiler::compile_type_to_c`
Thus caused buggy type tests in programs

Fixes #307
Signed-off-by: Jean Privat <jean@pryen.org>