Preassembly should be idempotent and extensible

[bgyori]

Currently it is not fully safe to run preassembly multiple times on the same set of statements. It is also not safe to add new statements to an already preassembled set and run preassembly again. In some cases when attempting to do this, cycles are introduced in the supports/supported_by graph. Making sure that these operations are safe would allow for more flexibility in structuring assembly pipelines.