Allow more than 16KB to be encoded

[layderv]

As per your README, the tool does not allow to store more than 16KB (header included).

How do you see this changed? A simple u16->u64 change? Is a version dump required (I'd say so)?

[woodruffw]

Yeah, we should bump STEG86_VERSION and consequently STEG86_MAX_CAPACITY_BYTES and STEG86_HEADER_SIZE_BITS.

I'd suggest changing it to just u32 instead of u64 for now, since it's unlikely that we'll see any real-world x86 binaries that come even close to exceeding 2.4GB of steganography capacity 🙂

[woodruffw]

(Also, it'd be ideal if we could preserve compatibility with the old header in the codebase. But if you look into it and it's a hassle, we can just spit out an error telling people to use an earlier version.)