Add frozen_string_literal: true to all files

[jhawthorn]

Also remove any encoding: UTF-8 comments which should no longer have any effect.

[tvdeyen]

Nice. Out of curiosity did you test how much memory actually gets saved on a regular store?

[jhawthorn]

Nice. Out of curiosity did you test how much memory actually gets saved on a regular store?

I haven't, and I'm not sure what exactly I would measure (maybe run stack_prof or memory_prof on a full checkout?). I'm not expecting a major improvement (but would love to be surprised by one).

[gmacdougall]

I don't expect that memory improvements will be the major win here, but consistent behaviour between this and the upcoming Ruby 3.0 is definitely desired.