Fix the following issue:
SHL opcode not implemented
I'm running some Solidity-compiled bytecode through smol-evm and it bails out partway with an UnknownOpcode error. Disassembling the bytecode I can see it's hitting a SHL (shift left) — and looking through opcodes.py I notice SHR is implemented but SHL isn't there at all.
Could we add it? The Solidity compiler emits SHL pretty routinely (bit packing, struct layout, masking, etc.), so right now smol-evm can't really get through most real contract bytecode. Behavior should match what the yellow paper / EVM spec defines for SHL.