rustup-dist: Use Download notifications to track install

[kinnison]

People have requested some indication of progress for long-running
install steps. This commit re-uses the download tracker logic to
provide a progress bar (the length is the compressed component
tarball but should be good enough) to provide such feedback.

Fixes: #1557

[kinnison]

All the rebase does is remove one of the fixes messages because it was over-enthusiastic.

[nrc]

Thanks for the PR, I think this is probably a good idea. I would move the reader from the manifestation module to somewhere else though, maybe the dist::download module or a utils module somewhere

[kinnison]

I will look at the right place to move it to, and attempt to get to it later when I'm home for the evening. I worry that the callback nesting might take me a while to get right though, so it might be a day or two before I can post depending on how awake I am when I get home. Thanks for the confirmation that the idea itself is okay though.

[kinnison]

@nrc turns out that the FileReaderWithProgress moved nicely to rustup_utils but I'm not entirely clear on how I might add tests for this. Obviously I've run it by hand and am confident it does what I wanted, but do you feel it needs something under cargo test ?

[kinnison]

Interestingly there's some formatting differences in the CI. I'll have to double-check my rustfmt install.

[kinnison]

I've rebased and hopefully that'll clear up that formatting issue.

[nrc]

Thanks!