Rule for by-name right associative operators

Warnings for the issue presented in SI-1980.  The rule code is the same as that
for the -Xlint warning with the same name.
Rule for nullary overrides.

Warns when a nullary method is overriden by a non-nullary one.
Merge branch 'master' of https://github.com/Lewix/scala-abide
Adapt lint rules to new package structure