Report the correct filename for client exceptions

... by passing the filename to eval.
make `reference`, `term`, `path` instance methods as y2r expects

Note that it also expects import and copy to be class methods,
that should be unified.

The transpiled code still does not work because "include YCP" is
missing in YCP::FooClass and YCP::Clients::FooClient. Before it
worked because constant lookup considers parent scopes.