Add proptest Arbitrary trait implementation

[GauravPawar101]

Implements proptest::arbitrary::Arbitrary for SmallVec<Item, INLINE, Global> behind a new proptest feature flag.

Changes

• Cargo.toml: Added proptest feature (requires std)
• src/lib.rs: Added Arbitrary impl using proptest::collection::vec
• tests/proptest.rs: Three property tests

Notes

• Feature requires std (as dev-time only)
• Incompatible with allocator-api2 — proptest generates std::vec::Vec while allocator-api2 uses allocator_api2::vec::Vec; the impl is gated

Closes #755

[GauravPawar101]

accidentally pushed ran rust fmt and it reformatted

[alejandro-vaz]

thanks @GauravPawar101

[GauravPawar101]

Welcome