Nullability handling improvements, including fixes #31

First two commits were required solely to set up my workflow and make IDE shut up, so feel free to revert them, if you want.

Also includes fix to bug with restoring Character/Byte variables (the existence of bug itself being another proof that _nobody_ uses those nowdays).
I see. `isAssignable` causes a bug with char/bytes and ints. I guess a test should be added to cover that edge case.