Stylistic pass and deps updates

[JohnDowson]

This prepares the crate for introduction of multiple tree kinds, as well as ridding it of weird import Golang-isms :)

[JohnDowson]

Please don't hesitate to tell me to improve PR description, it's the kind of thing I very much struggle coming up with. :)

[ambuc]

Hi @JohnDowson,

I noticed that you made a few kinds of changes in this PR. Some of your changes were purely stylistic, and others were logical and prepared the crate for further changes.

Could you please take out these two changes as separate PRs? I will almost certainly approve the logical changes immediately, since they will be short and easy to understand. I will probably approve the formatting ones quickly also, but I want to think harder about whether or not I can use a presubmit hook to auto format this crate. (I don't want to waste energy thinking about formatting style in reviews.)

[JohnDowson]

@ambuc there are config options for rustfmt, ones most relevant to this PR are probably imports_granularity and group_imports.