[memory usage] re-write union-find non-recursively

[fangism]

hackt/src/Object/inst/instance_alias.tcc

Line 777 in e77f05b

this->next = &*this->next->find();

has appeared in a stacktrace in a large design that stack-overflowed during the load of an elaborated .haco-c file (happened to be in cosim). Rewriting that function should greatly reduce stack usage.