Use Box<[InstructionBox]> in Executable

[dima74]

enum Executable {
    Instructions(Vec<InstructionBox>),
    Wasm(WasmSmartContract),
}

Vec<InstructionBox> in some cases allocates an initial buffer with size 4 (#5083 (comment)). InstructionBox is a large type (176 bytes), so it is worth replacing it with a read-only Box<[InstructionBox]> to be sure we don't waste memory on unused buffer capacity.