Thunk leak fixes

The strictness introduced in this commit was motivated by observing
thunk leaks in the eventlog2html output.

- Refactor attach instances list comprehension to avoid large
  intermediate thunks
- Refactor some HTML backend list comprehensions to avoid large
  intermediate thunks
- Avoid thunks accumulating in documentation types or documentation
  parser
- A lot of orphan NFData instances to allow us to force documentation
  values