Add the hash datatype

[u9g]

Adds `hash`: `["hash", { "alg": String, "type": Type, "body": Type }]`. Writing serializes the value as `body`, hashes the bytes with `alg` and writes the digest as `type`; reading yields the digest. It is a one-way field for protocols that carry a checksum of something in place of the thing itself, such as Minecraft's `HashedSlot` (a CRC32C per item component since 1.21.5).

`crc32` and `crc32c` are integer digests; when `type` is signed the hash is written in two's complement. Other algorithms are whatever the platform's crypto provides and produce raw digest bytes, so `type` is then a fixed-length `buffer`.

Schema, datatype reference and docs. Implementation: ProtoDef-io/node-protodef (PR to follow, linked below).

Context: PrismarineJS/prismarine-item#184 (comment)