Stable type identifiers: a missing piece for crABI and export

ParaDevOne:

Paths in Rust are always unique, by definition.

This is not true, as a simple example:

[dependencies]
synv1 = { package = "syn", version = "1" }
synv2 = { package = "syn", version = "2" }

(The --crate-name flag passed to rustc is essentially arbitrary, you could pass the same thing for every one of hundreds of dependencies and still get a working build out).