Add support for smallvec v1

[isharma228]

This adds the smallvec_1 feature which enables serialization and deserialization support for SmallVec from the smallvec crate.

SmallVec is supported with all serde_as conversions like DisplayFromStr, and works with any array size and element type.

The implementation required manual SerializeAs/DeserializeAs trait implementations because SmallVec's type parameter is an array type (A: Array) rather than an element type, which doesn't match the existing generic macro patterns.

[codecov]

Codecov Report

❌ Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.80%. Comparing base (ec0ffa3) to head (c83c4fc).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
serde_with/src/de/impls.rs	86.95%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #895      +/-   ##
==========================================
+ Coverage   67.74%   67.80%   +0.06%     
==========================================
  Files          36       36              
  Lines        9509     9537      +28     
==========================================
+ Hits         6442     6467      +25     
- Misses       3067     3070       +3     

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[jonasbb]

Thank you for the PR. I see how the Array type complicates the implementation a bit and makes it incompatible with the existing macros.

I performed two small changes. I sorted the features in Cargo.toml alphabetically, and I removed the alloc dependency as it is unnecessary to compile.