Continuously publish Cargo builds

Continuously publish Cargo builds

This commit tweaks Cargo's automation to continuously publish builds on Travis
an AppVeyor. Once this is merged we can hopefully turn off all buildbot
automation related to Cargo and purely rely on Travis and AppVeyor for this
repository. All CI matrices are ported over to Travis and AppVeyor and a new
musl build of Cargo is even added just to test out adding that for a spin.

Currently Cargo will upload the final artifact for each target to a directory
keyed by the commit hash to a new bucket on S3, rust-lang-cargo-dev. Once we're
happy with the builds then we can change this to `rust-lang-cargo` or anything
else at that point.