Conversion checking tweak

Slightly more liberal if there are holes in the terms being checked, so
that we don't fail too early.
Merge branch 'master' of github.com:idris-lang/Idris-dev
Fix for matchProblems

Needed to take into account that later matches could affect earlier
problems.
Store metavariables in ibc files

There's really no reason not to, and it means you can import something
unfinished without having to wait for it to typecheck every time.