Fix for stable Rust

[jbit]

Hi!

Thanks for writing this crate! It got me up and running with my HX711 based scale on a slow and ancient Raspberry Pi Zero. So far all readings have been very stable and correct.

I generally try to stick with the stable Rust compiler, so I noticed this crate uses an unstable feature (#![feature(negative_impls)]) which requires the nightly compiler.

I read through the code and understand you added the !Sync negative trait bound to prevent sharing of Hx711 between threads.

However, as far as I can tell, this is not required with the code in it's current state:

• All functions require a &mut self, which Rust guarantees is singular. (Thus preventing any reentrancy issues)
• Because the code uses blocking SPI, no transfers persist between calls.
• If the SPI HAL implementation is not Send+Sync safe, it will propagate correctly to the Hx711 struct
• No unsafe code exists in the library, which usually makes it very hard to not be Send+Sync safe

This pull request removes the !Sync bound, reformats the code using cargo +nightly fmt, and has some minor cleanups and typo fixes.

Please let me know if you have any questions or want to discuss anything 😄

Cheers!

--James

[crjeder]

Thanks jbit! Your insight into the compiler guarantees is highly appreciated! Great that you were able to confirm that the code is reentrant / thread safe. You found a bunch of awful typos. The code must have been a nightly one.. 😊