Rollup merge of #156204 - asder8215:pathbuf_into_string, r=nia-e

Implemented `PathBuf::into_string`

Noticed from this [Zulip chat](https://rust-lang.zulipchat.com/#narrow/channel/327149-t-libs-api.2Fapi-changes/topic/.E2.9C.94.20PathBuf.3A.3Ainto_string.3F/with/500962843) that there's been a pretty old [ACP](https://github.com/rust-lang/libs-team/issues/307) on introducing `PathBuf::into_string` that hasn't been implemented yet. I haven't seen anyone link a tracking issue to that [ACP](https://github.com/rust-lang/libs-team/issues/307), so I assume there have been no PRs to complete that ACP.

I made a tracking issue for this ACP [here](https://github.com/rust-lang/rust/issues/156203).