[breaking] Change internal model serialization to UBJSON. (#7556)

* Use typed array for models.
* Change the memory snapshot format.
* Add new C API for saving to raw format.

tests/ci_build/conda_env/cpu_test.yml

@@ -32,6 +32,7 @@ dependencies:
 - awscli
 - numba
 - llvmlite
+- py-ubjson
 - pip:
   - shap
   - ipython                     # required by shap at import time.

tests/ci_build/conda_env/macos_cpu_test.yml

@@ -31,6 +31,7 @@ dependencies:
 - jsonschema
 - boto3
 - awscli
+- py-ubjson
 - pip:
   - sphinx_rtd_theme
   - datatable

tests/ci_build/conda_env/win64_cpu_test.yml

@@ -18,3 +18,4 @@ dependencies:
 - jsonschema
 - python-graphviz
 - pip
+- py-ubjson

tests/ci_build/conda_env/win64_test.yml

@@ -16,3 +16,4 @@ dependencies:
 - python-graphviz
 - modin-ray
 - pip
+- py-ubjson