hc: Update to 2018 edition

[timotree3]

PR summary

This PR

• Updates hc's Cargo.toml to use the 2018 edition of Rust (74f04b2)
• Fixes the broken use declarations caused by the changes to namespacing in the 2018 edition (74f04b2)
• Removes most extern crates because the 2018 edition makes them unneeded (74f04b2)
• Deletes the remaining #[macro_use] extern crates and adds use declarations for their macros where needed (1693930)
• Removes the lint annotation #[warn(unused_extern_crates)] because there are no longer any extern crate declarations (50c68b0)
  • Why do we have this on any of our crates? Warning on unused extern crates should be the default...
• Does some small re-arranging of use declarations for clarity (fc1b594)

testing/benchmarking notes

N/A

followups

I'm currently working on one of these for holochain_core_types as well.

changelog

• if this is a code change that effects some consumer (e.g. zome developers) of holochain core, then it has been added to our between-release changelog with the format

- summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

documentation

• this code has been documented according to our docs checklist