Simplify imports by importing fewer items directly

[mgeisler]

When reading the code without an IDE, I find it useful to use explicit
module names, especially when items come from the standard library.

So io::Error instead of just Error, especially when people have
just been told about std::error::Error as well.

I also omitted most single-use items: I find it has less cognitive
overhead to say “we import fmt” and then later use fmt::Display
and fmt::Formatter in the code. It’s clear from the name that these
two things have something to do with formatting.

Finally, I made a few usages more consistent so that we refer to each
item in the same way within a single codeblock.