Better AST Printer

[onehr]

Now crust just use a simple AST Printer, which uses the depth as its parameter to control its indention level, but should be improved by ignoring some node information which are not useful for us to observe (cause it can be overwhelming sometimes) , but also keeps its scope level.

[onehr]

Maybe try to generate CFG from AST node to better visualize the structures of the AST.

https://doc.rust-lang.org/1.0.0/graphviz/index.html