cargo-tree: Compress feature nodes

[ehuss]

With the -e features flag, the list of features can be somewhat noisy. It might be useful to compress the display so that features that have only one incoming edge from another feature could be combined with others of the same. For example:

├── syn feature "derive"
│   └── syn feature "default" (*)
├── syn feature "parsing"
│   └── syn feature "default" (*)
├── syn feature "printing"
│   └── syn feature "default" (*)
├── syn feature "proc-macro"
│   └── syn feature "default" (*)

could be displayed as:

├── syn feature "derive", "parsing", "printing", "proc-macro"
│   └── syn feature "default" (*)