Implement tree model dump with code generator. (#4602)

* Implement tree model dump with a code generator.

* Split up generators.
* Implement graphviz generator.
* Use pattern matching.

* [Breaking] Return a Source in `to_graphviz` instead of Digraph in Python package.


Co-Authored-By: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

include/xgboost/feature_map.h

@@ -7,6 +7,8 @@
 #ifndef XGBOOST_FEATURE_MAP_H_
 #define XGBOOST_FEATURE_MAP_H_
 
+#include <xgboost/logging.h>
+
 #include <vector>
 #include <string>
 #include <cstring>