[ issue 1293 ] Cosmetic changes
[ issue 1293 ] Show instance not used any more when reporting errors.

The instance is

  instance Show a => Show (Position' (Maybe a))

from Agda.Syntax.Position.
[ issue 1293 ] Added TODO
[ issue 1293 ] Replaced show function by pretty
Merge remote-tracking branch 'my-agda/issue-1293'

Conflicts:
	src/full/Agda/TypeChecking/Errors.hs