Add a way to pretty-print the dependency tree

[gnzlbg]

Currently I need to:

cargo install cargo-tree
cargo tree

to pretty-print the dependency tree - which on first install takes a while, and because of cargo-tree dependency on cargo, needs to often be recompiled when Rust is upgraded.

I shouldn't need to install a cargo subcomman to print the whole dependency tree of a crate in a human readable form. Feels like a way too common task that deserves upstream support.