Prepare for a stable 1.0.0 release

[hcho3]

• Update dmlc-core submodule to the latest version, to take advantage of various optimizations in the logging facility.
• Lower CMake requirement to 3.13. See Implementing Rust runtime API #189. The FetchContent_MakeAvailable command is only for CMake 3.14+, so define it ourselves when CMake version is 3.13.
• Make static libs (libtreelite.a, libtreelite_runtime.a) optional to build. Add CMake option BUILD_STATIC_LIBS to build static libs. This is useful for building a Conda package, since Conda recipes should not build static libs for most use cases.
• Fix DLL loading for Python 3.8+ on Windows. See LoadLibrary for Windows + Python 3.8 doesn't seem to search through PREFIX/Library/bin conda-forge/python-feedstock#307
• Some CI plumbing. For some reason, Homebrew GCC fails export_lib() with a cryptic error, and I cannot reproduce on my machine. For now, the CI should just use Apple Clang. Some tests that are known to fail with Apple Clang are marked as XFAIL.

[hcho3]

@dovahcrow Would you be able to review this? Let me know if this PR is adequate for your Rust runtime.

[dovahcrow]

@hcho3 tbh I'm not very familiar with C++ and its build system. I can only confirm this is sufficient for building the rust runtime binding.

[hcho3]

Somehow the CI is unable to use Homebrew GCC :( Why did the CI have to break at this time, when 1.0 is about to go out ;)

[dovahcrow]

Somehow the CI is unable to use Homebrew GCC :( Why did the CI have to break at this time, when 1.0 is about to go out ;)

Just let me know if there's anything I can help with.

I just finished nearly all the bindings of the runtime in Rust, except for reading DMatrix from file and CSR.

[codecov-io]

Codecov Report

Merging #246 (7514d43) into mainline (792514a) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##             mainline     #246      +/-   ##
==============================================
- Coverage       83.69%   83.69%   -0.01%     
  Complexity         44       44              
==============================================
  Files              93       93              
  Lines            6932     6937       +5     
  Branches           42       42              
==============================================
+ Hits             5802     5806       +4     
- Misses           1106     1107       +1     
  Partials           24       24              

Impacted Files	Coverage Δ	Complexity Δ
python/treelite/core.py	82.14% <100.00%> (+2.14%)	0.00 <0.00> (ø)
runtime/python/treelite_runtime/predictor.py	86.63% <100.00%> (+0.14%)	0.00 <0.00> (ø)
src/compiler/ast_native.cc	96.49% <0.00%> (-0.21%)	0.00% <0.00%> (ø%)

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