Refactor InMemory Stuff -> {InMemory,FileSystem}Storage

[charlespwd]

Follow up on #145

From

class InMemoryTemplate < Template; end
class InMemoryJsonFile < JsonFile; end
class InMemoryTheme < Theme; end

to

class Storage; end
class InMemoryStorage < Storage; end
class FileSystemStorage < Storage; end

class Theme
  def initialize(storage); end
end

class Template
  def initialize(name, storage); end
end

class JsonFile
  def initialize(name, storage); end
end

[macournoyer]

Beautiful refactoring 👏

Just a few optimization ideas.

[macournoyer]

Instead we should just reset @lines in write, this way lines will always be the correct method to call:

def write
  # ...
  @lines = nil
end

[charlespwd]

Not sure I understand this one. The intention behind this method was to do a check on source != updated_source before writing. And to use it in tests rather than the path.read we had in test_helper.

[charlespwd]

Now that we're back at one. Can probably inline a couple of those vars.