Remove unreachable errors and prevent unreachable panics from being generated

[izik1]

Breaking changes:

• Error::{InvalidData, BufferUnderflow} no longer exist.

Possible concerns:

• removes the #[deny(unsafe_code)] due to using core::hint::unreachable_unchecked.
• the scoping of the unsafe might need changing.
• no benchmark to prove potential speed gains.
• no cargo bloat run to say how much this would impact binary sizes.

Considered alternatives:

• don't do this.
• do this, but use unreachable! instead of unreachable_unchecked.
  • this wouldn't require unsafe, but it would compile panics that are still provably unreachable.

Rationale:

• Removes unproducable error types, which means less errors to handle.
• likely slight performance gain due to code that's easier to optimize.
• slight binary size decrease due to less panics.
• less panics are nice, even if they're unreachable.

closes #2

[GabrielMajeri]

Thanks for the PR! Indeed it seems that it's impossible for Rust code to pass us invalid UTF-8 strings (or at the very least, it's not our responsability to handle).