[xbase][typesystem] Improved lambda inference

Lambdas can also be used to implement generic methods,
e.g. 

interface IReadAccess<P> {
  <T> T exec(IUnitOfWork<T, P> ouw)
}

Java8 does not allow to implement those with lambda
expressions.

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=447516

Change-Id: If0043faf80f26f4ba3b69eb5d3bf4f3092b4a6cd