Improve error messaged by dropping untagged enums

Untagged enums do not provide good error messages and likely never will,
given that there are multiple PRs which are just completely ignored
([serde#2376](serde-rs/serde#2376) and
[serde#1544](serde-rs/serde#1544)).

Instead using `content::de` the untagged enums can be replaced by custom
buffering. The error messages for `OneOrMany` and `PickFirst` now look
like this, including the original failure for each variant.

```text
OneOrMany could not deserialize any variant:
  One: invalid type: map, expected u32
  Many: invalid type: map, expected a sequence
```

```text
PickFirst could not deserialize any variant:
  First: invalid type: string "Abc", expected u32
  Second: invalid digit found in string
```

The implementations of `VecSkipError` and `DefaultOnError` are updated
too, but should not result in any visible changes.

serde_with/CHANGELOG.md

@@ -7,6 +7,26 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+### Changed
+
+* Improve the error message when deserializing `OneOrMany` or `PickFirst` fails.
+    It now includes the original error message for each of the individual variants.
+    This is possible by dropping untagged enums as the internal implementations, since they will likely never support this, as these old PRs show [serde#2376](https://github.com/serde-rs/serde/pull/2376) and [serde#1544](https://github.com/serde-rs/serde/pull/1544).
+
+    The new errors look like:
+
+    ```text
+    OneOrMany could not deserialize any variant:
+      One: invalid type: map, expected u32
+      Many: invalid type: map, expected a sequence
+    ```
+
+    ```text
+    PickFirst could not deserialize any variant:
+      First: invalid type: string "Abc", expected u32
+      Second: invalid digit found in string
+    ```
+
 ## [2.3.1] - 2023-03-10
 
 ### Fixed

serde_with/Cargo.toml

@@ -65,6 +65,8 @@ chrono_0_4 = {package = "chrono", version = "0.4.20", optional = true, default-f
 doc-comment = {version = "0.3.3", optional = true}
 hex = {version = "0.4.3", optional = true, default-features = false}
 indexmap_1 = {package = "indexmap", version = "1.8", optional = true, default-features = false, features = ["serde-1"]}
+# The derive feature is needed for the flattened_maybe macro.
+# https://github.com/jonasbb/serde_with/blob/eb1965a74a3be073ecd13ec05f02a01bc1c44309/serde_with/src/flatten_maybe.rs#L67
 serde = {version = "1.0.122", default-features = false, features = ["derive"]}
 serde_json = {version = "1.0.45", optional = true, default-features = false}
 serde_with_macros = {path = "../serde_with_macros", version = "=2.3.1", optional = true}