Support nocov comment at the end of a line

[cyberdelia]

Support nocov comments at the end of a line to mark it as irrelevant, it's a bit less verbose than blocks to ignore some benign things, i.e:

case value
when String
  StringFilter
when Array
  ArrayFilter
else
  raise "Absurd case" # :nocov: 
end

[cyberdelia]

Fixed the build failure in #1069.

[cyberdelia]

Freshly rebased!

cc/ @PragTob

[will]

@amatsuda please consider this pull request