aim_datatypes: move list links to reduce valgrind false positives
aim_datatypes: move list links to reduce valgrind false positives

Valgrind doesn't consider interior pointers to imply reachability. Instead, it
will mark these blocks as possibly lost. Moving the list links to the beginning
of the allocation makes Valgrind happy.

jnealtowns/infra,jnealtowns/infra,jnealtowns/infra