…rate-background

* origin/kve/layout-writer:
  Introduce LayoutWriter, for easier custom layouts without needing to use the Layout protocol.
  Bumping versions to 0.18.0
  Some changes to the ElementPreview type to use it within Market's preview element.
  Add `AccessibilityContainer.identifier`

* origin/main:
  Bump redcarpet from 3.5.0 to 3.5.1
  Prep 0.19.1
  Update CHANGELOG
  Revert "Merge pull request #190 from square/kve/validate-elements-are-structs"
  Prep 0.19.0
  Remove Travis builds
  Set up Github Actions
  Add a test for how often the builder is called
  Code review, add a couple methods that I used in validating the behaviour of negative positions
  Validate elements are structs
  Fix empty environment referencing a forever-living MeasurementCache.
  Fix SPM build

* main:
  Bump versions to 0.21.0
  Add additional ways to create and mutate an overlay, eg to ease conditional construction.
  Introduce conditionals on Element
  Prep 0.20.0
  Add `Transformed` element