Add RcBuf variant to DataPayload

[sffc]

See #667

[sffc]

I added a new variant to DataPayload for buffers and wired it up in FsDataProvider and StaticDataProvider. I used the following bound on functions where the data struct needs to be deserializable:

for<'de> <M::Yokeable as Yokeable<'de>>::Output: serde::de::Deserialize<'de>,

Good news:

• The core library code compiles
• I did some interesting stuff to make SerdeDeDataProvider compile, including a double-callback function
• Somehow, I am using the proper attach_to_cart and it compiles

Bad news:

• Call sites of FsDataProvider and StaticDataProvider do not compile, giving errors such as "the trait for<'de> serde::de::Deserialize<'de> is not implemented for <DecimalSymbolsV1 as Yokeable<'de>>::Output"

I believe this is rust-lang/rust#85636 again. Unfortunately, I don't think I can use the "reference workaround" here again, because I care about Self (like I do with Clone).

@Manishearth Thoughts?

UPDATE: Manish answered my question and posted the solution in #85636.

[codecov-commenter]

Codecov Report

Merging #816 (904bd08) into main (a7a9650) will increase coverage by 0.11%.
The diff coverage is 62.90%.

@@            Coverage Diff             @@
##             main     #816      +/-   ##
==========================================
+ Coverage   75.32%   75.43%   +0.11%     
==========================================
  Files         192      193       +1     
  Lines       12374    12499     +125     
==========================================
+ Hits         9321     9429     +108     
- Misses       3053     3070      +17     

Impacted Files	Coverage Δ
experimental/provider_static/src/lib.rs	4.00% <0.00%> (ø)
ffi/capi/src/provider.rs	0.00% <0.00%> (ø)
provider/core/src/dynutil.rs	38.88% <ø> (ø)
provider/core/src/serde.rs	47.05% <0.00%> (-12.95%)	⬇️
utils/yoke/src/lib.rs	100.00% <ø> (ø)
utils/yoke/src/trait_hack.rs	0.00% <0.00%> (ø)
utils/yoke/src/yokeable.rs	81.81% <ø> (-18.19%)	⬇️
provider/fs/src/deserializer.rs	37.50% <52.63%> (-3.41%)	⬇️
provider/core/src/data_provider.rs	70.31% <54.54%> (-3.28%)	⬇️
provider/core/src/erased.rs	82.88% <85.36%> (+15.74%)	⬆️
... and 14 more

---

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[coveralls]

Pull Request Test Coverage Report for Build f0b79ddad05faf2b5d1be5e40ef90cd90c1e61e4-PR-816

• 78 of 124 (62.9%) changed or added relevant lines in 9 files are covered.
• 21 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.1%) to 75.508%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
experimental/provider_static/src/lib.rs	0	1	0.0%
ffi/capi/src/provider.rs	0	3	0.0%
utils/yoke/src/trait_hack.rs	0	4	0.0%
provider/core/src/data_provider.rs	6	11	54.55%
provider/core/src/erased.rs	35	41	85.37%
provider/fs/src/deserializer.rs	10	19	52.63%
provider/core/src/serde.rs	0	18	0.0%

Files with Coverage Reduction	New Missed Lines	%
experimental/provider_ppucd/src/support.rs	1	58.49%
provider/fs/src/deserializer.rs	1	37.5%
provider/fs/src/fs_data_provider.rs	1	89.83%
components/uniset/src/provider.rs	2	52.94%
tools/datagen/src/main.rs	2	33.33%
utils/yoke/src/yokeable.rs	2	81.82%
provider/core/src/dynutil.rs	12	38.89%

Totals
Change from base Build bac781dee970092ae7b953e08ae9f9c69a267524:	0.1%
Covered Lines:	9557
Relevant Lines:	12657

---

💛 - Coveralls

[Manishearth]

Overall looks pretty good! Glad to see this coming together so nicely.

[Manishearth]

suggestion: Please add a comment with the actual bound we are trying to express here (and a link to the issue)

(ditto everywhere else the hack wrap is used)

[sffc]

There are 4 places and I made them all look like this:

    // Actual bound:
    //     for<'de> <M::Yokeable as Yokeable<'de>>::Output: serde::de::Deserialize<'de>,
    // Necessary workaround bound (see yoke docs):
    for<'de> YokeTraitHack<<M::Yokeable as Yokeable<'de>>::Output>: serde::de::Deserialize<'de>,

[sffc]

I added yoke::trait_hack with docs and examples.

[sffc]

Docs preview on yoke::trait_hack: http://icu4x-pr-artifacts.storage.googleapis.com/gha/288eaad6aa65ea075ce58d0a463a05293ca4fe09/docs/yoke/trait_hack/index.html

[Manishearth]

small nit, r=me otherwise