Adds the magic `frozen_string_literal: true` comment to every file and
enables a Rubocop rule to make sure that it's always going to be there
going forward as well.
See here for more background [1], but the basic idea is that unlike many
other languages, static strings in code are mutable by default. This has
since been acknowledged as not a particularly good idea, and the
intention is to rectify the mistake when Ruby 3 comes out, where all
string literals will be frozen. The `frozen_string_literal` magic
comment was introduced in Ruby 2.3 as a way of easing the transition,
and allows libraries and projects to freeze their literals in advance.
I don't think this is breaking in any way: it's possible that users
might've been pulling out one of are literals somehow and mutating it,
but that would probably not have been useful for anything and would
certainly not be recommended, so I'm quite comfortable pushing this
change through as a minor version.
As discussed in #641.
[1] https://stackoverflow.com/a/37799399
I wanted to see what fixing Rubocop TODOs was like, so I tried to
eliminate all the easy ones. Most of these were pretty easy, and the
changes required are relatively minimal.
Some of the stuff left is harder. Pretty much everything under
`Metrics/*` is going to be a pretty big yak shave. A few of the others
are just going to need a little more work (e.g. `Style/ClassVars` and
`Style/GuardClause`). Going to stop here for now.
