Add stack of elaboration context to ElaboratingArg

This is for argument-specific error reflection.
Remove some commented-out code
Merge remote-tracking branch 'upstream/master' into unifier-context

Conflicts:
	src/Idris/ElabTerm.hs
Add application context to all error messages

This makes it easier to find errors. It will also enable improved error
reflection soon.
Re-enable function arg error reflection

It now uses the same code path as normal error reflection.
Fix test
Merge pull request #990 from david-christiansen/unifier-context

Add more context to unification errors internally