Optimize Witness Serialization

[DanGould]

fix #942

self.to_vec() allocates, it should be possible to avoid it - just feed the items into serializer.

based on #1068

[apoelstra]

cargo +nightly build seems to fail -- I think you need to feature-gate the new serde stuff.

[tcharding]

GitHub does not let me comment on the commit log in line so commenting here.

The commit log is not correctly formatted. The subject should be separated from the body by a newline. (How are you making commits? Your tool should be able to be configured to warn you of this.)

I'm pretty anal about commit logs, I think they are important, they last for ever so are worth doing well. Here is the canonical blog post on the topic that I always link to: https://cbea.ms/git-commit/

I would prefer to see something like (notice subject then body (WHY then WHAT):

Optimize Witness serialization

Currently we allocate a new vector when serializing a Witness, this is inefficient and unnecessary. We can use serialize_seq to feed the witness elements directly into the serializer.

Optimize Witness serialization by removing the allocation.

[DanGould]

I see the one import was already gated and wrongly assumed that would apply to every line that came after. this might work at the top as well:

#[cfg(features = "serde")]
use serde::{self, ser::SerializeSeq};

but I matched the imports with @tcharding's Human readable pr so as not to conflict.

This was supposed to be trivial merge so I don't have to ask for ci runtime, but I learned about feature gates and why git rebase reword highlighted red on me. fixed.

[Kixunil]

ACK 9bf9591

(under the condition CI succeeds)

Thanks!

[apoelstra]

ACK 9bf9591

[apoelstra]

The commit message is fixed BTW to use Tobin's suggestion.

[tcharding]

Thanks @DanGould.