Split `ptr_uint64` in two

[ssoelvsten]

There are some spicy invariants when using the ptr_uint64 class; namely the out_idx, flag, and the need to use essential(...). If we somehow can make a source<...> type and target<...> type in arc (similar to uid) that enforces this, then the type checker would be much better at helping the programmer.

• Add arc::source_type which takes a uid and an out_idx (none of which is defaulted).
• Add arc::target_type which takes a uid and a flag (flag defaulted to false).

Additional Context
Requested by Erik Funder Carstensen