using optional + ref as the return type of to* and supported heterogeneous lookup for string

[wbpcode]

Signed-off-by: wangbaiping wbphub@live.com

[codecov-commenter]

Codecov Report

Base: 85.98% // Head: 86.42% // Increases project coverage by +0.43% 🎉

Coverage data is based on head (b176576) compared to base (a60bf21).
Patch coverage: 81.48% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   85.98%   86.42%   +0.43%     
==========================================
  Files          25       25              
  Lines        2269     2195      -74     
==========================================
- Hits         1951     1897      -54     
+ Misses        318      298      -20     

Impacted Files	Coverage Δ
hessian2/object.hpp	76.95% <76.74%> (+0.81%)	⬆️
hessian2/basic_codec/class_instance_codec.cc	95.74% <100.00%> (-0.09%)	⬇️
hessian2/basic_codec/list_codec.cc	79.54% <100.00%> (-0.19%)	⬇️
hessian2/basic_codec/map_codec.cc	84.07% <100.00%> (-0.28%)	⬇️
hessian2/basic_codec/object_codec.cc	87.96% <100.00%> (ø)

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[wbpcode]

Hi, @zyfjeff could you take a look when you have free time? Thanks.
This PR unified the return type of to* methods to optional ref wrapper.

And heterogeneous lookup for map with key type of StringObject is added. By this way, we can reduce unnecessary memory allocations when doing map search by raw string.

[zyfjeff]

reference: #3

[zyfjeff]

LGTM