chore(*): normalise copyright headers (#4497)
This diff makes sure that all files start with a copyright header
of the following shape
```
/-
Copyright ...
... Apache ...
Author...
-/
```
Some files used to have a short description of the contents
in the same comment block.
Such comments have *not* been turned into module docstrings,
but simply moved to a regular comment block below the copyright header.
Turning these comments into good module docstrings is an
effort that should be undertaken in future PRs.
