Add model delta tests for use-site variance
Fix errors in model delta test samples

Currently, the errors don't affect the delta, but that's going to change
soon (see #1152).

Almost all the changes are changing formal toplevel functions to proper
functions (with the definition '{}' or '=> nothing;'); the only other
case is the test for annotation aliasing, which failed due to
ceylon/ceylon-spec#1081. (See also #1152.)
Detect error changes as model changes

See #1152.
Add tests for error model deltas

See #1152.
Merge pull request #1153 from ceylon/errorModelDelta

Detect error changes as model changes