[Breaking] Remove dmlc-core dependency

[chyunsu3]

Closes #244

This PR will be divided into smaller PRs for easier review.

Marking this PR as "breaking," per discussion in #284

[codecov]

Codecov Report

Merging #285 (bf3e612) into mainline (4dd95c4) will increase coverage by 0.01%.
The diff coverage is 33.33%.

@@              Coverage Diff               @@
##             mainline     #285      +/-   ##
==============================================
+ Coverage       85.10%   85.11%   +0.01%     
  Complexity         46       46              
==============================================
  Files             106      106              
  Lines            8014     8020       +6     
  Branches           50       50              
==============================================
+ Hits             6820     6826       +6     
  Misses           1169     1169              
  Partials           25       25              

Impacted Files	Coverage Δ
runtime/java/treelite4j/src/native/treelite4j.cpp	66.85% <ø> (ø)
src/annotator.cc	91.47% <ø> (ø)
src/c_api/c_api.cc	97.09% <ø> (ø)
src/compiler/ast/build.cc	98.14% <ø> (+1.78%)	⬆️
src/compiler/ast/dump.cc	0.00% <ø> (ø)
src/compiler/ast/fold_code.cc	88.46% <ø> (+1.66%)	⬆️
src/compiler/ast/is_categorical_array.cc	100.00% <ø> (+7.14%)	⬆️
src/compiler/ast/load_data_counts.cc	100.00% <ø> (+9.09%)	⬆️
src/compiler/ast/quantize.cc	100.00% <ø> (+1.88%)	⬆️
src/compiler/ast/split.cc	100.00% <ø> (+2.27%)	⬆️
... and 11 more

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[chyunsu3]

Removed irrelevant changes, to make this PR smaller.

[chyunsu3]

I've split this PR into smaller chunks: #287, #288, #289, #290, #291, #292, #293.