refactor extapi

[bjjwwang]

• refactor extapi
• fix some bugs in translateGep

[codecov]

Codecov Report

Merging #1283 (8fe3974) into master (a17e605) will decrease coverage by 0.01%.
Report is 6 commits behind head on master.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1283      +/-   ##
==========================================
- Coverage   64.46%   64.45%   -0.01%     
==========================================
  Files         223      223              
  Lines       23813    23815       +2     
==========================================
  Hits        15351    15351              
- Misses       8462     8464       +2     

Files	Coverage Δ
svf/lib/MemoryModel/AccessPath.cpp	3.09% <0.00%> (-0.04%)	⬇️

... and 3 files with indirect coverage changes