Fix scale-codec for Multihash

[koushiro]

waiting for #139 to be merged firstly

This PR is a breaking change.

BREAKING CHANGE: Write the digest of multihash directly to dest when encoding multihash, since we have known the size of digest.

We do not choose to encode &[u8] directly, because it will add extra bytes
(the compact length of digest) at the beinging of encoding result.
For a valid multihash, the length of digest must equal to size field of multihash.
Therefore, we can only read raw bytes whose length is equal to size when decoding.

And the digest of Multihash<U64> is of type [u8; 64], taking Code::Sha2_256 as an example,
if the digest is treated as &[u8] when encoding, useless data will be added to
the encoding result.

[koushiro]

@vmx PTAL

[vmx]

@koushiro I know very little about the scale-codec, I haven't really used it. Could you perhaps put some information into the commit message why it is a breaking change?

[koushiro]

Could you perhaps put some information into the commit message why it is a breaking change?

Updated, and you could compare the difference between the encoding results of multihash in test_scale before and after the change

[koushiro]

@vmx What do you think? Any suggestions?

[koushiro]

Example:

let mh1 = Multihash::<crate::U32>::wrap(Code::Sha2_256.into(), Sha2_256::digest(b"hello world").as_ref()).unwrap();

let mh3: Multihash<crate::U64> = Code::Sha2_256.digest(b"hello world");

Before:

mh1: code = 18, size = 32, digest = [185, 77, 39, 185, 147, 77, 62, 8, 165, 46, 82, 215, 218, 125, 171, 250, 196, 132, 239, 227, 122, 83, 128, 238, 144, 136, 247, 172, 226, 239, 205, 233] (0xb94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9)
Multihash<U32> scale-codec encoding: 120000000000000020b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9

mh3: code = 18, size = 32, digest = [185, 77, 39, 185, 147, 77, 62, 8, 165, 46, 82, 215, 218, 125, 171, 250, 196, 132, 239, 227, 122, 83, 128, 238, 144, 136, 247, 172, 226, 239, 205, 233] (0xb94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9)
Multihash<U64> scale-codec encoding: 120000000000000020b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde90000000000000000000000000000000000000000000000000000000000000000

After:

mh1: code = 18, size = 32, digest = [185, 77, 39, 185, 147, 77, 62, 8, 165, 46, 82, 215, 218, 125, 171, 250, 196, 132, 239, 227, 122, 83, 128, 238, 144, 136, 247, 172, 226, 239, 205, 233] (0xb94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9)
Multihash<U32> scale-codec encoding: 120000000000000020b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9

mh3: code = 18, size = 32, digest = [185, 77, 39, 185, 147, 77, 62, 8, 165, 46, 82, 215, 218, 125, 171, 250, 196, 132, 239, 227, 122, 83, 128, 238, 144, 136, 247, 172, 226, 239, 205, 233] (0xb94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9)
Multihash<U64> scale-codec encoding: 120000000000000020b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9

[vmx]

Sorry for the delay, and thanks for providing an example, that was really helpful.

[koushiro]

@vmx Could you release a new version?

Maybe the new version can include #135

[vmx]

@koushiro would it be OK if I do a release next week? I'd like to also get #138 in if possible (if it isn't ready by next week, I'll do a release without it).

[koushiro]

would it be OK if I do a release next week? I'd like to also get #138 in if possible (if it isn't ready by next week, I'll do a release without it).

Ok, thanks!

[vmx]

@koushiro sorry for the delay. 0.15 is now released.