Auto merge of #114443 - tgross35:cstr-len, r=dtolnay

Implement `cstr_count_bytes`

This has not yet been approved via ACP, but it's simple enough to get started on.

- ACP: rust-lang/libs-team#256
- Tracking issue: rust-lang/rust#114441

`@rustbot` label +T-libs-api

Author: bors