collections: write wincode vec elements via schema to match reader

[plutohan]

Fixes #196.

The wincode SchemaWrite impls for TrailingVec and the prefixed-vec wrappers wrote elements as a raw byte copy sized by size_of::<T>(), while the read side decodes each element via its schema. For non-POD T those sizes disagree and the parse boundary drifts, so the field after the vector decodes wrong.

Both impls now write and size each element through SchemaWrite, matching the reader. No wire-format change for POD types. Added non-POD round-trip regression tests with a trailing field to pin the parse boundary.

[joncinque]

@febo can you take a look?

[febo]

Thanks for the PR @plutohan! I left a comment on the issue since supporting non-pod types might affect how JS handles these types. Let's wait to see what @lorisleiva says about it.

[febo]

Looks good to me. Just left a few nits.

[febo]

nit: We could use try_fold here.

Suggested change

	// Sum the serialized size of each element, matching the per-element
	// decoding performed by the read side.
	let mut expected_size = 0usize;
	for item in src.0.iter() {
	expected_size = expected_size.saturating_add(<T as SchemaWrite<C>>::size_of(item)?);
	}
	// Sum the serialized size of each element, matching the per-element
	// decoding performed by the read side.
	let expected_size = src
	.0
	.iter()
	.try_fold(0usize, |size, item| -> WriteResult<usize> {
	Ok(size.saturating_add(<T as SchemaWrite<C>>::size_of(item)?))
	})?;

[febo]

nit: Same here, we can use try_for_each.

Suggested change

	for item in src.0.iter() {
	<T as SchemaWrite<C>>::write(&mut writer, item)?;
	}
	Ok(())
	}
	src.0
	.iter()
	.try_for_each(|item| T::write(&mut writer, item))
	}

[febo]

nit: Same as above, try_fold.

Suggested change

	let mut expected_size = core::mem::size_of::<$prefix_type>();
	for item in src.0.iter() {
	expected_size = expected_size
	.saturating_add(<T as SchemaWrite<C>>::size_of(item)?);
	}
	let expected_size = src
	.0
	.iter()
	.try_fold(core::mem::size_of::<$prefix_type>(), |size, item| -> WriteResult<usize> {
	Ok(size.saturating_add(<T as SchemaWrite<C>>::size_of(item)?))
	})?;

[febo]

nit: Same as above, try_for_each.

Suggested change

	for item in src.0.iter() {
	<T as SchemaWrite<C>>::write(&mut writer, item)?;
	}
	Ok(())
	src.0
	.iter()
	.try_for_each(|item| T::write(&mut writer, item))

[plutohan]

Applied, thanks!

[febo]

Looks great, thanks!