Add -Winferred-safe-imports warning

Authored by phadej on May 6 2019, 3:14 PM.

Description

Add -Winferred-safe-imports warning

This commit partly reverts e69619e923e84ae61a6bb4357f06862264daa94b
commit by reintroducing Sf_SafeInferred SafeHaskellMode.

We preserve whether module was declared or inferred Safe. When
declared-Safe module imports inferred-Safe, we warn. This inferred
status is volatile, often enough it's a happy coincidence, something
which cannot be relied upon. However, explicitly Safe or Trustworthy
packages won't accidentally become Unsafe.

Updates haddock submodule.