ENH: Use memoryview to open reader in MMAP storage backend

[ChengjieLi28]

What do these changes do?

Related issue number

Fixes #xxxx

Check code requirements

• [ ] tests added / passed (if needed)
• [ ] Ensure all linting tests pass

[codecov[bot]]

Codecov Report

Merging #675 (ec66cfb) into main (81dea1d) will increase coverage by 6.91%. The diff coverage is 94.11%.

@@            Coverage Diff             @@
##             main     #675      +/-   ##
==========================================
+ Coverage   86.66%   93.57%   +6.91%     
==========================================
  Files        1026     1026              
  Lines       79812    79839      +27     
  Branches    16563    16564       +1     
==========================================
+ Hits        69166    74712    +5546     
+ Misses       9098     3442    -5656     
- Partials     1548     1685     +137     

Flag	Coverage Δ
unittests	93.47% <94.11%> (+6.90%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
python/xorbits/_mars/storage/mmap.py	97.97% <92.85%> (-2.03%)	:arrow_down:
python/xorbits/_mars/storage/core.py	96.22% <100.00%> (+0.07%)	:arrow_up:

... and 108 files with indirect coverage changes