test/dyn_ops.c includes the header and calls most of what it declares, so a
renamed function fails to link. It does not catch a signature that drifts
while the name stays -- C links on names, not types -- and it does not reach
flan_dyn_map_get, flan_dyn_map_set, flan_dyn_map_contains, flan_dyn_is_nil or
flan_dyn_need_not_nil at all, none of which dyn_ops.c mentions. The comment
now says that, and says which hole is closable from here.