a4004d47

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Log skipped DisableMutations modules clearly

Previously 'added 0 mutations' was printed for both genuinely-empty
modules (e.g. autoexporter stubs) and modules opted out via
{-# ANN module ("DisableMutations" :: String) #-}.  That made the
log noisy and gave the impression the plugin was doing work.

Now annotated modules print a single 'mutation: skipping <module>
(DisableMutations)' line and the typecheck action returns immediately
without walking the AST or looking up splice spans.