Set up testing of every stdx crate

[brson]

As part of stdx's validation mission, it should be testing that the exact crate revisions it is promoting really do work on all tier-1 platforms.

Probably the way to do this is to set up another crate inside the repo called stdx-validate (or something) that has a build script that generates a custom test suite. This test suite will parse the stdx Cargo.toml to retrieve the exact version of every crate in stdx, download those crates, frob them to make them work correctly, and run their test suites.

This test suite should probably also verify that, if the crate has a dependency on another crate in stdx, it is tested with the version specified by stdx, not some arbitrary version.

Then set up CI for tier 1 platforms.

[brson]

The CI should also use rustup to install the exact toolchain stdx claims compatibility with.

[brson]

@alisha17 has been working on this.