Support new CRATES syntax

[thesamesam]

cargo.eclass now supports @ as a delimiter between package name and version as it's faster to parse and is already common in Rust world.

Also, ${CARGO_CRATE_URIS} should be used in SRC_URI instead now.

References:

• https://marc.info/?l=gentoo-dev&m=168718685909185&w=2
• https://marc.info/?l=gentoo-dev&m=167597571925550&w=2
• https://marc.info/?l=gentoo-dev&m=168694323423232&w=2

[Kangie]

on it