Add invariants checks at various points

[kaashif]

Invariants are:

• All cards still exist and are still with their original players (not lost by being drawn and not eliminated/cycled)
• No invalid cards were played to the path/battleground (although there are some Hobbits with weird rules here that may make this non-trivial but still fairly straightforward)
• Items are played to valid wielders and no-one else
• Maybe more

Could check this with a fuzzer or just running random game for a while and logging any exceptions.