Encourage space-leak correct value storage

[ndmitchell]

We've had a bunch of variables which ended up storing thunks - see haskell/ghcide#772 for merely the latest example. We should:

• Introduce safe wrappers around modifyVar_ and modifyMVar_ named modifyVar_' and modifyMVar_', which do an evaluate before storing.
• Switch all our code to using these wrappers.
• Use HLint to ban the lazy variants.

[jneira]

That sounds to be a relatively easy job (thanks to @ndmitchell instructions)