parsing: remove potential panics

[rmsyn]

Removes the unwrap calls in BigEndian*::from_bytes implementations.

Currently, the implementation will never panic on the unwrap call, because the prior get call always returns the appropriate length byte slice.

However, it is safer to call .ok()? to bubble-up the None result. Which, again, will never be returned with the current implementation. The function will bubble-up None on the get call before reaching the try_into.

[repnop]

I think I'm going to close this as it better describes the intent that this will never panic, and at opt-level = 3 they both produce identical results, thanks for the PR though!

[rmsyn]

I think I'm going to close this as it better describes the intent that this will never panic

I have reviewed and used a number of libraries that make this same assumption to disastrous results.

There are enough places where implicit panics can crash Rust programs. Removing and safe-guarding against panics, making panic-safe Rust, is a very important part of hardening Rust software. This is especially true of no_std software that is likely being used in kernels, firmware, and/or bare-metal programs.

Your project though, so best of luck.