Allow getting and setting options from IDESlave

This supports checkboxes in Emacs for show implicits and error
context.
Fix "Force" in case tree for new indexed form
Merge pull request #1006 from david-christiansen/ideslave-opts

Allow getting and setting options from IDESlave
Use postulates for decEq primitive proofs

This means we can assert there is a proof at compile-time, but we're not
going to rely on the structure of that proof anywhere (which can cause
problems if they are relied on elsewhere).

Fixes #1005