[WIP] a simple mappedfile wrapper

[nihui]

z-image-ncnn 1024x1024

end2end	7900xtx
baseline	38.35s
+mmap	31.86s

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.14%. Comparing base (7d5d32d) to head (acbf440).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6537      +/-   ##
==========================================
+ Coverage   92.94%   93.14%   +0.19%     
==========================================
  Files         809      847      +38     
  Lines      256714   266560    +9846     
==========================================
+ Hits       238613   248289    +9676     
- Misses      18101    18271     +170     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.