Fix the nullptr warning in red/black tree impl.
Merge pull request #244 from layus/feature/nullptr

Fix the nullptr warning in red/black tree impl.
Update deprecated uses of Tcl_Interp::result
Merge pull request #245 from layus/feature/interp->result

Update deprecated uses of Tcl_Interp::result
Implement OS.system
Merge pull request #246 from layus/feature/os.system

Implement OS.system