Merge #128

128: Remove unstable offset_from feature r=torfmaster a=torfmaster

# Summary

In this PR I remove the unstable `offset_from` feature which is used to calculate the number of bytes of a slice of bytes in a given range of addresses (the flash region). Clearly, the feature provides a safe and concise way of computing this. However, I find that using a minimal number of unstable/experimental features here has a higher value.

Co-authored-by: torfmaster <briefe@kebes.de>