More efficient state encoding - combine multiple flags into one variable

[georgwiese]

Motivation: Consider this test: For a single loadbu, the chip materializes flags__{0,1,2,3}_0. They come from OpenVM's load/store flags, encoding the (opcode, byte_offset). The opcode is known at compile time and for the byte offset, there are only 4 possibilities. Therefore, there are only 4 possible assignments for the 4 flags. So, we could replace them with 2 binary columns and build a degree-2 expression for each of the 4 cases.

[chriseth]

Maybe related: #3083