Move fringe icon into separate function
Make error indication customizable
Replace association list with a simple cl-case
Simplify hook setup and teardown
Get rid of deferred checking

This deferred checking stuff was just a super-complicated way to check
only visible buffers.  So let's just do this right away: Inhibit all
syntax checks in invisible buffers and recheck buffers if they become
visible again.
Unbreak Rubocop tests

Grr, @bbatsov ;)
Fix byte compiler warning in Rubocop tests
Get rid of superfluous warning