Explicit instruction_length

[pgoodman]

Right now things look at bit like this: What it should look like is that there is a named advice node called instruction_length, and then each DecoderResult node should have an extra condition, e.g. instruction_length == 5, and then we should instead see (add rip instruction_length) rather than (add rip 5). Anywhere where there is a use of NEXT_PC in remill should then rely on the (add rip instruction_length) as the value for NEXT_PC.