Whether unparsers are generated or not has implications on legality
checks for the grammar, so it makes sense for this to be a property of
the grammar itself: introduce an Lkt annotation as well as a
langkit.parsers.Grammar argument for it, and remove the corresponding
CompileCtx argument.
Also rename "generate_unparser" to "generate_unparsers" for consistency
across the codebase.
Transition all relevant testcases to it. In the process, port
docstrings/comments from test.py files to expected_concrete_syntax.lkt
(now test.lkt) sources and fix stylechecks issues there.
This test fails sometimes because it makes no sense at all. There is no
information about rewriting context being equivalent when closing and
reopening a new one (see doc for 'langkit.rewriting.start_rewriting').
