Fixes idris-lang/Idris-dev#971. Renames induction holes such that they
are unique in context.
Add :apropos command to REPL

:apropos STR searches names, types, and docs for STR
Add :apropos to CHANGELOG
Merge pull request #976 from ahmadsalim/bug/inductionnamefix

Fixes idris-lang/Idris-dev#971.
Merge pull request #978 from david-christiansen/feature/apropos

Add :apropos command to REPL
Rename UnusedArgs to Erasure.
Remove export of traceUnused.
Simplify unused arg search.
Cleanup and purification in Erasure.hs.
More purification.
Simplification with pattern guards.
findUsed stub written.
Make ProjCase take exactly one alternative.   (TODO: is this safe?)
Intercommit. (Deeper usage analysis.)
Ctor usage analysis roughly complete.
Intercommit.
Another intercommit.
Reapply 08779298ce2e95b3e16a5a3e5d9c0cf3936d0507 from master.
Intercommit.
Redefine dep analysis data structures.