hash improvements

[ab9rf]

• add a hasher for df::coord2d
• provide a binding so Lua code can obtain hash values for objects that are hashable

[SilasD]

Note that Lua uses 64-bit integers, so will prefer a 64-bit hash.
I think that map coordinates are always less than 1024,
(in a 16x16 map, x and y can be 0..767)
so can be stuffed into a 32-bit hash with e.g. (x << 20) & (y << 10) & z
as long as it's guaranteed that none of those are negative, I guess.
((x & 0x3FF) << 20 & ((y & 0x3FF) << 10) & (z & 0x3FF), maybe.