Support RE workspaces (non-incremental)

towards MR3 of #985 (closed)

• benchmark: https://gitlab.com/snippets/1915283
  • non-ws
  • open-ws
• refactor test

there are some remaining optimization issues which require mechanisms such as diff trees, in particular there will be a performance regression in preparing sandboxes in most cases (except imports), see fixmes

closes #933 (closed)