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

[fangism]

https://github.com/fangism/hackt/blob/e77f05bd2382f40c9493b86b9b32ef9a009b3263/src/Object/inst/instance_alias.tcc#L777

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.