core_types: Update to 2018 edition

[timotree3]

PR summary

This PR

• Updates holochain_core_types's Cargo.toml to use the 2018 edition of Rust (ca02f6c)
• Fixes the broken use declarations caused by the changes to namespacing in the 2018 edition (ca02f6c)
• Removes most extern crates because the 2018 edition makes them unneeded (ca02f6c)
• Deletes the remaining #[macro_use] extern crates and adds use declarations for their macros where needed (88af2fe)
• Updates some lifetime parameters to use the new Foo<'_> notation in accordance with 2018 edition idioms (575c348)

testing/benchmarking notes

N/A

followups

I plan to make more of these for all crates that aren't 2018 in this mono-repo.

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

[timotree3]

CI is failing because the use declarations that I added for the serde derives aren't working on that machine. I added use serde::{Deserialize, Serialize};, which works on my machine, but it seems that in general for that to work you need to enable the derive feature of serde.

That leaves two options:

• Replace these instances of serde::{Deserialize, Serialize} with serde_derive::{Deserialize, Serialize}
• Enable the derive feature of serde, removing the need for the serde_derive dependency altogether.

Personally, I prefer the second approach, as it turns code that looks like

use serde::{Deserialize, Serialize};
use serde_derive::{Deserialize, Serialize};

#[derive(Deserialize, Serialize)]
struct Foo { /* ... */ }

into just

use serde::{Deserialize, Serialize};

#[derive(Deserialize, Serialize)]
struct Foo { /* ... */ }

I'll be pushing a commit that uses said approach.

[timotree3]

The current CI failure seems to be cause by a regression in structopt-derive v0.3.6. I'm looking into it now.

[timotree3]

I opened TeXitoi/structopt#315. In the meantime, I suggest we mitigate this by adding an unused structopt-derive dependency with the same version as our structopt dependency. I'll push a commit that does this now.

[timotree3]

As a matter of fact, the breakage in structopt-derive v0.3.6 should be affecting the develop branch too. I'll make a PR applying the same mitigation I did here.