Use attohttpc instead of reqwest for fetching

[charlespierce]

Info

• As originally found by @joshuarli in Investigate build bloat #536, we don't need all the async features of reqwest in Volta, since all of our HTTP access is synchronous.
• Unfortunately, while reqwest provides a synchronous API, it still uses the async engine and task executor (tokio) under the hood.
• Compiling that bloats our build times and executable size, by bringing in an entire async executor when we don't actually use it.
• attohttpc is a purely-synchronous HTTP Request client with a very similar API to reqwest.

Changes

• Removed reqwest and added attohttpc to all of the appropriate crates.
• Upgraded hyperx to 1.0.0, to match the transitive dependency on http
• Updated the code to fit attohttpcs slightly different API

Notes

• This results in a ~25% reduction in the executable size for volta, and a ~30% reduction in the executable size for volta-shim.
• Anecdotally, this reduced the time for a clean cargo build --release by ~ 22% (from 6m12s down to 4m50s).
• For performance, again anecdotally, I see no noticeable difference for fetching a version of Node, though I do get a statistically significant speedup on the hot path (likely a result of the smaller binary).

[joshuarli]

Oh neat, I didn't know at all about hyperx back when I made a shitty attempt at porting old header stuff to new header stuff to upgrade reqwest. Seems like it made things a lot easier.

[charlespierce]

Yeah, definitely, the hyperx stuff made it a lot easier, not having to re-implement the header parsing. I didn’t know about the crate either until a day or two ago 😄

[chriskrycho]

⚡ ship it! Nice clean refactor, and cutting down compile time and build size both? ⚡ ❗