Efficient type-level bytestring serialization.

[Natural]s have a convenient syntax, and we can use them as a type-level bytestring by asserting that each Natural is <=255 when reifying. This module provides type classes which give you a serializer for a given [Natural].

We maximize efficiency by grouping bytes into machine words. We have to be pretty verbose to achieve this. Each type class attempts to group bytes into its machine word type, and if it can't (i.e. not enough bytes remain), it hands off to the next type class which handles the next smaller machine word.

I did a quick Core check and found that GHC seems to successfully generate minimal code for this e.g. for an 8-byte magic, GHC will do one writeWord64OffAddr# of a constant. Great!

The only way I can think of to make this faster is to somehow obtain an Addr# with a known length. With that, we could memcpy. But that would be slower for small magics, and maybe others. And I doubt we can conjure up an Addr# at compile time. So I'm fairly confident that this is the best you're gonna get.