Tracking issue for Treelite 3.9 and 4.0

[hcho3]

As discussed in #438, we are spinning off the C codegen ("compiler") part of Treelite into a separate project (dubbed tl2cgen).

Goals

• Keep Treelite small. It should do one thing well, namely serve as a universal exchange and serialization format for decision tree forests.
• Hand off the control of tl2cgen to other contributor(s), as I do not have bandwidth to maintain it in the future.
• Make it easy for other projects to adopt Treelite as serialization format.
• Future-proof Treelite 4.0 by making breaking changes that will enable additional types of decision tree forests.

Functions to be migrated to tl2cgen + removed from Treelite 4.0

• treelite.Annotator
• treelite.Model.compile
• treelite.Model.export_lib
• treelite.Model.export_srcpkg
• treelite.generate_makefile
• The entire treelite_runtime package will be renamed to tl2cgen_runtime and will be moved to the tl2cgen project.
  • treelite_runtime.DMatrix
  • treelite_runtime.Predictor
• The entire Java runtime will be moved to tl2cgen project.

Create new project tl2cgen

• [x] Create new repository dmlc/tl2cgen
• [x] Set up directory skeleton for C++ source files
• [x] Set up CMake build
• [x] Set up directory skeleton for Python package
• [x] Set up pyproject.toml and PEP 517 backend
• [x] Set up pytests. In particular, there should be integration tests to ensure that C codegen functions of Treelite can be replaced 1-to-1 with those in tl2cgen.
• [x] Make dmlc/tl2cgen public
• [x] Migrate relevant issues from dmlc/treelite to dmlc/tl2cgen
• [x] Set up a working Sphinx doc
• [x] Create a 0.1 release for tl2cgen
• [x] Publish tl2cgen to PyPI
• [x] Submit a pull request to Conda-forge's staged recipes repo

Release Treelite 3.9

• [x] Write a migration guide.
• [x] Add deprecation warnings to Model.export_lib and other related functions. Warnings should direct users to the migration guide.
• [x] Publish Treelite 3.9 to PyPI and Conda-forge

Release Treelite 4.0

• [x] Remove the functions in the list above.
• [x] Address the blockers (see below).
• [x] Create tl2cgen 0.2 to handle Treelite 4.0+ serialization format.
• [x] Publish Treelite 4.0 and tl2cgen 0.2 to PyPI and Conda-forge

Tasks for Treelite 4.0

• [x] #489
• [x] #503
• [x] #515
• [ ] #469
• [x] Breaking change: #463
• [x] Breaking change: #445
• [x] Breaking change: #467
• [x] #446
• [x] #429
• [x] #426
• [x] Breaking change: #468
• [x] #492
• [ ] #517