Refactor CMake build; create treelite_runtime Python pkg

[hcho3]

• Now git submodules are removed.
• Runtime gets a separate Python package: treelite_runtime.
• Revamp CMake build script using Modern CMake paradigm.
• Rewrite setup.py.

[codecov-commenter]

Codecov Report

Merging #167 into master will increase coverage by 75.93%.
The diff coverage is 83.84%.

@@              Coverage Diff              @@
##             master     #167       +/-   ##
=============================================
+ Coverage      9.12%   85.06%   +75.93%     
- Complexity       54       56        +2     
=============================================
  Files            58       58               
  Lines          3681     3642       -39     
  Branches         47       47               
=============================================
+ Hits            336     3098     +2762     
+ Misses         3317      515     -2802     
- Partials         28       29        +1     

Impacted Files	Coverage Δ	Complexity Δ
include/treelite/compiler.h	100.00% <ø> (+90.00%)	0.00 <0.00> (ø)
include/treelite/compiler_param.h	100.00% <ø> (ø)	0.00 <0.00> (?)
include/treelite/predictor.h	84.37% <ø> (ø)	0.00 <0.00> (?)
include/treelite/tree.h	95.00% <ø> (+87.50%)	0.00 <0.00> (ø)
src/c_api/c_api_runtime.cc	100.00% <ø> (ø)	0.00 <0.00> (?)
src/compiler/native/pred_transform.h	86.20% <ø> (+86.20%)	0.00 <0.00> (ø)
src/frontend/protobuf.cc	88.12% <ø> (+87.65%)	0.00 <0.00> (ø)
src/model.cc	100.00% <ø> (ø)	0.00 <0.00> (?)
src/optable.cc	100.00% <ø> (ø)	0.00 <0.00> (?)
src/predictor/thread_pool/spsc_queue.h	89.18% <ø> (ø)	0.00 <0.00> (?)
... and 59 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f5c0cb8...dc8c1f2. Read the comment docs.