Always parse "to" as a scalar. No need for a branch; it's left over from when I was trying to enforce canonical RLP (in which case "to" must be 0 or 20 bytes).
The old code would be wrong if we had multiple txns per proof, as if to=0 we wouldn't write that field to memory, so it could have an old value from a previous txn.
Uses a variant of Dijkstra's, with a few pruning mechanics, to find a path of instructions between the two stack states. We don't explicitly store the graph though.
The Dijkstra implementation is somewhat inspired by the `pathfinding` crate. That crate doesn't quite fit our needs though.
If we need to make it faster later, there are a lot of allocations and clones that we could probably eliminate.
- `GET_STATE_ROOT` and `SET_STATE_ROOT` deal with the root of the state trie, and will be called from storage routines. Similarly `GET_RECEIPT_ROOT` and `SET_RECEIPT_ROOT` deal with the root of the receipt trie.
- `PANIC` enables an unsatisfiable constraint, so no proof can be generated.
- `GET_CONTEXT` and `SET_CONTEXT`, used when calling and returning
- `CONSUME_GAS` charges the sender gas; useful for cases where gas calculations are nontrivial and best implemented in assembly.
- `EXIT_KERNEL` simply clears the CPU flag indicating that we're in kernel mode; it would be used just before a jump to return to the (userspace) caller.
- `MLOAD_GENERAL` and `MSTORE_GENERAL` are for reading and writing memory, but they're not limited to the main memory segment of the current context; they can access any context and any segment. I added a couple macros to show how the they would typically be used.
There may be more later, but these are the ones I think we need for now. I tried to fill in smaller invalid sections of the decoder's tree, as Jacqui suggested, while keeping related opcodes together. We can fine tune it when the opcode list is more stable.
These are all intended to be priviledged, i.e. they will be treated as invalid if used from userspace, for compatibility as well as (in some cases) security reasons.
