We must ensure that the crates.io version has all the functionality the internal one does. Apparently the only thing missing is https://github.com/rust-lang/rust/blob/master/src/libserialize/serialize.rs#L748-L815

Then we just need to change the deps in the Cargo.tomls and remove the crate.

Motivation is that I want save-analysis to depend on an external crate which in turn depends on rustc-serialize (the crates.io version). (We can't use Serde because of complex dependencies on the proc macro crate). That means we will pull in the crates.io version, so we may as well just have that, rather than two versions kicking around (and two sources of truth, etc.).

cc #40177