Safety + Miri

[Wicpar]

• removed all unchecked variants as they were slower than the new ones (~10-30%), and unsafe for no reason (~10-30%)
• replaced most code with safe functions
• Miri now passes all checks
• disabled env logger for miri as it is unable to introspect.
• replaced indices, ranges and sizes with usize, converting from u8 is not free, and there is no reason to use a u8 as it takes a full register anyway. Ranges are checked anyway too. This is also for API coherence with the rest of the rust ecosystem. Ranges are checked anyway too.

[Wicpar]

there is a good case to be made to use the replace_range function to solve insert, push, remove and their variations

[paulocsanz]

Wow, this is so much cleaner and safer! Thank you!

I will have to test this as we don't currently have CI, but it seems incredible. I will do it after work.

Don't mind my comments too much, I can make those changes later, although I was curious about changing &str for &[u8] in places where we expect utf8.

Thanks again, this is super awesome, you improved this library so much!