SourceNatConfig could store infallible block numbers instead of fallible port pairs

So.... I'm wondering if rather than first_port and last_port this can just take the multiple of NUM_SOURCE_NAT_PORTS. Since NUM_SOURCE_NAT_PORTS is 2^14 and the number of ports is 2^16, there are only 4 possible values. So this could actually take an enum, say, SourceNatPortBlock { Block0, Block1, Block2, Block3 }. That would make this constructor infallible.

Then you can serialize the data in that fashion as well, and deserialization won't require further validation.

Some places may have a port pair, and you can have a constructor that converts that into a SourceNatPortBlock, returning the below error. But places that don't have the raw ports won't have to deal with errors.

Not sure how much it increases work for you, maybe in a followup/after r8 if it disrupts this work too much?

Originally posted by @sunshowers in https://github.com/oxidecomputer/omicron/pull/5560#discussion_r1571610090

Start by locating SourceNatConfig, NUM_SOURCE_NAT_PORTS, and the first_port/last_port usages in the repository; no file or test is named in the issue. Read the discussion linked from pull request #5560, then trace serialization, deserialization, and port-pair conversion. Done means the four valid blocks are represented infallibly and all affected callers and validation paths are updated.