add GraphPrintTree

[vc60er]

add GraphPrintTree, can print like this:

dependence tree:
┌── target(int=123)
├── wait(int=123)
├── timeout(int=123)
├── path_string(string=path string)
├── path_strings([]string=[path1 path2])
├── path_map(map[string]int=map[path1:1 path2:2])
├── test(*main.Test=0xc0000bc240)
│    ├── target(int=123)
│    ├── timeout(int=123)
│    ├── path_string(string=path string)
│    ├── path_strings([]string=[path1 path2])
│    └── path_map(map[string]int=map[path1:1 path2:2])
└── dep(*main.Dep=0xc000096290)
     ├── test(*main.Test=0xc0000bc240)
     │    ├── target(int=123)
     │    ├── timeout(int=123)
     │    ├── path_string(string=path string)
     │    ├── path_strings([]string=[path1 path2])
     │    └── path_map(map[string]int=map[path1:1 path2:2])
     └── wait(int=123)

[teou]

good idea.
please add unit tests for this merge request @vc60er .