* core/asm: Fix encoding of pushed labels
EVM uses big-endian byte-order, so to pad a label value to 4 bytes,
zeros must be added to the front, not the end.
* core/asm: Fix PC calculations when a label is pushed
Incrementing PC by 5 is only correct if the label appears after a jump,
in which case there is an implicit push. When it appears after an explicit
push, PC should only be incremented by 4.
* core/asm: Allow JUMP with no argument
This way, a label can be pushed explicitly, or loaded from memory to
implement a jump table.
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