Add two new feature flags:

• unstructured: enabling this makes the StructuredScript type just a thin wrapper arround ScriptBuf. This is probably less memory efficient, but a lot more efficient performance wise: api_generate_partial_script reduces from 98.24s to 24.62s (~4x speedup)
• unchecked removes the validity check on opcodes, which further reduces the time to 16.58s (~6x speedup compared to structured). T

My local BitVM repo still compiles with both features enabled, but I left the features disabled by default to not break anything