Remove event_arrow from testsuite
META: be more friendly with the 'type_conv' syntax extension.
Small fix in META
fix after rebase
No more assumption on the deriving syntax to load ..

the user have to provide the syntax he wants to use.
We may provide option in eliom tools to select the syntax
but `package deriving.syntax.std` or `-pacakge deriving.syntax.tc` my be enough