2018-ification changes include:

• replacing extern crate imports (except for ones annotated with
  #[macro_use] to prevent touching more files; maybe in a follow-up)
• removing standalone imports; for example, use serde_json; -- the
  code already uses serde_json::* where necessary

Minor formatting changes include:

• collapsing imports from the same root (e.g. use std::io::Read; use std::io::BufRead -> use std::io::{BufRead, Read};
• separating crate imports from external imports from std imports --
  this is how many project seem to order their imports, most notably the
  official Rust repo
• moving some uses from the first line to below the file's doc comment
• changing file-level doc comments from /// to //! (so they actually
  show up as documentation)
• rustfmt messing with the nom macros in src/commentparser.rs

Most of the 2018-ification was done by cargo fix --edition-idioms, but
all changes were manually inspected to catch any detrimental changes,
and cargo test/cargo check still succeed.

Verified code impact (none) with cd ofborg && nix-shell ../shell.nix --run 'cargo check && cargo test'.

Supercedes #417.

Small note: when I ran cargo fix --edition-idioms, it introduced anonymous lifetimes (<'_>) to a few structs. I undid this change for the sole reason that I am still unfamiliar with lifetimes. rust_2018_idioms is the lint that notices this -- reason being: hidden lifetime parameters in types are deprecated. If this is desirable, I can add it back in (currently sitting in a git stash entry).