Proper pretty-printing of dependent pairs

 * Dependent pairs are properly highlighted as either constructors or
   types
 * Names bound in dependent pair types show up as bound in their scope
Highlight variables from error context as bound
Show error contexts with pretty-printer
Merge remote-tracking branch 'upstream/master' into semantic-pprint

Conflicts:
	src/Idris/Delaborate.hs
Merge pull request #865 from david-christiansen/semantic-pprint

Improve pretty-printing