rust: add cargo.
rust: add cargo.

This follows discussion on Homebrew/homebrew#33322, where it has been hypothesized that
the best course of action may very well be to include cargo in the rust
formula. Reasons include:

- upstream packages releases with cargo
- cargo has no formal versioning rules, merely binding a particular
  cargo nightly to a given rust release
- cargo needs a non feature-gated rust to build
- cargo needs to fetch cargo to bootstrap itself (at the make stage)

The most practical way to get the cargo version bound to a rust release
is to download a binary rust release and run cargo -V, which shows the
SHA1 it was built from. This is what has been done for the beta.

Alternatively, building with --HEAD relaxes all version constrains and
builds both from their respective master HEADs.

Curl is a dependency of .travis.install.deps.sh while both curl and
python are a dependency of make (via src/etc/dl-snapshot.py) to download
a cargo snapshot. Each one makes respective use of a src/rustversion.txt
and a src/snapshots.txt file, combined with some very lighteight
processing, to find the required downloads.

Thus it is be possible to include fetched downloads as resources to
improve cacheability and reduce the dependency profile as long as one
maintains consistency between each build component.

Closes Homebrew/homebrew#38674.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

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