Stepping in shrapnel fix and unification of shard code

As in title. Shard code was split between two files in completely different directories, so I've moved it all to code/game/objects/items/weapons/shards.dm

Fixes #5992.

If you'd like a version of this that leaves the code where it is, [I have one ready](https://github.com/GinjaNinja32/Baystation12/tree/shardfix); just ask and I'll make a PR for it instead of this one.