chore: make ExtBytecode pointer private

[rakita]

close #1902

[github-actions]

Valgrind Results:

==4481== Cachegrind, a cache and branch-prediction profiler
==4481== Copyright (C) 2002-2017, and GNU GPL'd, by Nicholas Nethercote et al.
==4481== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==4481== Command: target/release/revme bench snailtracer
==4481== 
--4481-- warning: L3 cache found, using its data for the LL simulation.
Running snailtracer example!
elapsed: 1.936055099s
==4481== 
==4481== I   refs:      462,304,056
==4481== I1  misses:          7,199
==4481== LLi misses:          4,062
==4481== I1  miss rate:        0.00%
==4481== LLi miss rate:        0.00%
==4481== 
==4481== D   refs:      213,298,506  (135,374,143 rd   + 77,924,363 wr)
==4481== D1  misses:        340,895  (    201,232 rd   +    139,663 wr)
==4481== LLd misses:        136,164  (      3,914 rd   +    132,250 wr)
==4481== D1  miss rate:         0.2% (        0.1%     +        0.2%  )
==4481== LLd miss rate:         0.1% (        0.0%     +        0.2%  )
==4481== 
==4481== LL refs:           348,094  (    208,431 rd   +    139,663 wr)
==4481== LL misses:         140,226  (      7,976 rd   +    132,250 wr)
==4481== LL miss rate:          0.0% (        0.0%     +        0.2%  )