Make binary installers for mac and linux

[brson]

We need the installation experience to be very easy everywhere. Currently we require unixes to build from source, but that shouldn't be strictly necessary - we already distribute snapshot binaries that work.

Add a new build target that creates a binary installer similarly to the snapshots.

[brson]

Nominating.

[catamorphism]

1.0, high

[lucab]

Debian is tracking rust status to try to provide a proper package when it's time (ie. not now), some notes are at https://wiki.debian.org/Teams/RustPackaging
Some additional packaging notes are at https://github.com/mozilla/rust/wiki/Note-packaging

[g5pw]

rust port is already part of MacPorts.

[flaper87]

What's the final decision here based on the latest discussion that happened on mailing list?

It'd be nice to have a list of things that should happen to consider this bug completed:

• tarballs generation (?)
• Windows binaries (?)
• OSx pkg files (?)

As mentioned in #12546 (and on the mailing list thread) I'd prefer not having any distro specific packaging support as part of Rust's build system. I think the community will take care good care of that.