fix: assign anonymous IDs to named entities

fix: assign anonymous IDs to named entities

Instead of using string pointers as IDs, we now assign the IDs for named
entities exactly the same way we assign them for anonymous identities. This
works because we first check the internal map before creating any new ID, so
processing the same name twice will result in the same ID as before.

This fixes #1767. Now clusters within a graph are consistently processed in the
order in which they appear in the input file, rather than an order dependent on
pointers given out by the allocator.