Deserialize struct variants from a sequence in Value

[yuxi-liu-wired]

Depends on #484: the first commit here is the Value commit of #484 (its Remaining helper is used by this fix). Review the last commit; once #484 is merged this PR reduces to it.

fix: reject extra array elements when deserializing from a Value

The Value deserializers (from_owned_value, from_borrowed_value and
the &Value variants) handed arrays to the visitor without checking
afterwards that every element was consumed, so a fixed-length type
silently dropped the rest:

let v = to_owned_value(b"[1,2,3]")?;
from_owned_value::<(u8, u8)>(v)          Ok((1, 2))
from_refowned_value::<[u8; 2]>(&v)       Ok([1, 2])

The text deserializer rejects these (fix/seq-extra-elements), as do
serde_json's from_value and sonic-rs. Arrays now go through
visit_array, which returns invalid_length when elements are left,
like serde's SeqDeserializer::end.

Test: value_fixed_length_sequences_reject_extra_elements (fails before
this change).

fix: deserialize struct variants from a sequence in Value

A struct variant written as a JSON array deserializes from text, but
from an owned or borrowed Value (from_owned_value etc.) it failed with
Unexpected(Some(Object), Some(Array)):

from_slice::<E>(br#"{"S":[5,"k"]}"#)                 Ok(S { a: 5, .. })
from_owned_value::<E>(to_owned_value(..)?)           Err(Unexpected(..))

struct_variant now hands an array to the visitor through
visit_array, which also rejects wrong lengths. serde_json (serde-rs/json#1049) and
sonic-rs had the same gap.

Builds on fix/value-seq-extra-elements (uses its visit_array).

Test: value_struct_variant_from_sequence (fails before this change).

Testing

Each commit adds a regression test that fails before the change. The full test suite passes and
cargo fmt --check is clean on the changed files.

Found with a differential fuzzer comparing serde_json, simd-json, sonic-rs and jiter.

[Licenser]

just some minor test addition

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.23%. Comparing base (c7d4fba) to head (cf629f7).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #488      +/-   ##
==========================================
+ Coverage   72.00%   75.23%   +3.23%     
==========================================
  Files          49       49              
  Lines       11347    11613     +266     
==========================================
+ Hits         8170     8737     +567     
+ Misses       3177     2876     -301     

Flag	Coverage Δ
unittests	75.23% <100.00%> (+3.23%)	⬆️

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