Bundler already checks checksums for the _downloaded_ checksum from
Rubygems.org against the file checksum of the downloaded package.

We additionally check a previously recorded checksum from a file against
the file checksum of the downloaded package

Also, stricter version of checking checksum. Bundler's method skips in
certain cases which we do not want

Deletes now-used plugin code

Also,

- Flatten Gemfile.checksum format
- Extract commands into classes
- Move binary to within gem
- Handle when remote checksum could not be found on rubygems.org
- No update task yet
- Cleanup with frozen_string_literal
- Organise requires better
- Fix commands to be simpler
- Also exit 1 if a checksum is unverified
- Make puts consistently stderr