Merge pull request #244 from layus/feature/nullptr

Fix the nullptr warning in red/black tree impl.
Merge pull request #245 from layus/feature/interp->result

Update deprecated uses of Tcl_Interp::result
Implement OS.system
Fix included libs for LLVM/Clang
Merge pull request #246 from layus/feature/os.system

Implement OS.system
Merge branch 'feature/fix-llvm-include' into features
Fix included libs for LLVM/Clang
Merge branch 'feature/fix-llvm-include' into features