rust-chg: update dependencies Differential Revision: https://phab.mercurial-scm.org/D8359

rust-chg: depend on released version of tokio-process Still it is futures-0.1 based. Differential Revision: https://phab.mercurial-scm.org/D8358

rust-chg: add brief comment about initial capacity of temp_sock_path() I don't know if it can be expressed as a compile-time constant, so it's a comment for now. About this series: This is quite old patches for rust-chg. I heard from Octobus people that there's a plan to do an experiment on merging hgcli + chg + some Rust?, so I decided to respin the rust-chg series. Maybe we'll rewrite the core to leverage the recent async/await functionality, but I want to first make my old patches in so the rust-chg can be a drop-in replacement for the chg of C. Compiler warnings will be removed later, and the codebase will be upgraded to the 2018 edition later. Differential Revision: https://phab.mercurial-scm.org/D8357