line: avoid Vec in Opcode
line: avoid Vec in Opcode

Otherwise we get a lot of extra baggage even when it isn't used.

Before:
test bench_executing_line_number_programs ... bench: 161,251 ns/iter (+/- 4,392)

After:
test bench_executing_line_number_programs ... bench: 116,779 ns/iter (+/- 4,008)

gimli-rs/gimli,gimli-rs/gimli