Fix unchecked warning.

Sorry, my schooling in warning suppression was deficient.
Taught -Xlint about @implicitNotFound.

And also about what a compiler identifier with a $ is likely
to look like. So it can stop burying me in warnings about missing
String interpolators which aren't so missing.
Eliminate a pile of -Xlint warnings.

Some unused private code, unused imports, and points where
an extra pair of parentheses is necessary for scalac to have
confidence in our intentions.
Eliminated the accumulated feature warnings.

No, this isn't busywork, how dare you suggest
such a thing. I intend my tombstone to say

           HERE LIES EXTEMPORE,
  WHO ELIMINATED A LOT OF SIP-18 WARNINGS
             REST IN PEACE
Merge branch 'master' into pr/warning-cleanup

Conflicts:
	src/compiler/scala/tools/nsc/typechecker/Contexts.scala
Merge pull request #2437 from paulp/pr/warning-cleanup

Quieting down a bunch of warnings.