isAfterTyper test

Add test whether we are after typer. Use it to assert that
eta expansion and implicit search do not happen after typer.
Avoid -Ycheck after errors
Better printing of type variables

Will now print the full chain of instances using ? for uninstantiated and ' for
instantiated.
Avoid checking for tpt non empty in implicit defs after typer.

The check assumes that inferred TypeTrees do not exist yet, but after
the typer they do exist.
Addiong an adapt to implicit parameters.

Implicit parameters were not adapted to the formal argument before which
caused a -Ycheck violation.

Todo: we should find out whether we can do without the adapt in typedImplicit whose result
is afterwards thrown away.
Making reporters configurable

Add method to install a new reporter in a subcontext's typer state.
Refining TreeChecker to work immediately after typer.

Making TreeChecker work immediately after typer. This requires a number of fixes.
Also, errors now cause exceptions instead of being reported on the command line.