This coder doesn't really have a format, it's more a way of defining a format. It should be flexible enough to work with many existing binary formats, and endianness configurability would need to be a part of that.

Performance is not really a concern for this implementation. I would guess that the indirection from calling through all the Swift protocol methods will completely swap any cost of having extra branches (which will be predicted correctly about 99.99% of the time if there's a lot of them).