Allow compressing hashes into a single `UInt` or `Vector{UInt}`

[kernelmethod]

Problem

The hashes returned by most hash functions tend to use a lot of memory. For instance, a length-0 Vector{Int64} (e.g. as returned by LpHash) is 40 bytes:

julia> Base.summarysize(Vector{Int64}(undef, 0))
40

Moreover, using these hashes as a key into a database or hash table is difficult since in general they may not understand the datatype being used for the key.

Proposed solution

The solution I'm proposing is to add a function compress_hash that accepts a Vector{<:Integer} or BitArray{1} and converts it into a UInt32, UInt64, or Vector{UInt8}.

• For instance, we could use Julia's built-in hash function and simple let compress_hash(x) = hash(x), which returns UInt64.
• Alternatively, we could reinterpret x as an Array{UInt8} and use sha256(x), which returns Vector{UInt8}.

Notes

• It's worth considering whether or not compress_hash needs to be cryptographically secure. I suspect that it should be in order to be on the safe side for various potential applications of this package. In that case, we will need to define a type such as

struct HashCompressor
    salt :: Vector{UInt8}
end

(hashfn::HashCompressor)(x::Vector{UInt8}) = hcat(hashfn.salt, x) |> sha256

• Adding on to the last bullet point: it may be worth looking at the new BLAKE3 as a fast alternative to sha256, though it's unlikely that we'll be hashing anything large enough to justify going to great lengths in order to do this.