Some of the `%stack` operations are now trivial, but I kind of like keeping `%stack` since it serves as documentation as well, making comments about the stack unnecessary.
Again borrowing syntax from NASM. Example from the test:
%macro spin
%%start:
PUSH %%start
JUMP
%endmacro
One thing this lets us do is create "wrapper" macros which call a function, then return to the code immediately following the macro call, such as
%macro decode_rlp_scalar
%stack (pos) -> (pos, %%after)
%jump(decode_rlp_scalar)
%%after:
%endmacro
I used this to clean up `type_0.asm`.
However, since such macros need to insert `%%after` beneath any arguments in the stack, using them will be suboptimal in some cases. I wouldn't worry about it generally, but we might want to avoid them in performance-critical code, or functions with many arguments like `memcpy`.
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.
