Member reference: pre and post conditions should be part of function type

[smillst]

When comparing the types of method references, the the pre- and post-conditions should be considered.

In contexts such as pseudo-assignments, the pre- and post-conditions should be checked like they are when checking method overriding. Although some may not be useful because post conditions generally refer to fields (but there will be no shared fields). See test case in checker/tests/nullness/java8/methodref/Postconditions.java.

[smillst]

Pre and post conditions are checked for lambdas either. See todo in org.checkerframework.common.basetype.BaseTypeVisitor#visitLambdaExpression.