Fix warnings

[zachs18]

Fixes unused_unsafe warnings on Rust >=1.92.0 by adding #[allow]s since the relevant code is not safe in the MSRV 1.83.0 declared in Cargo.toml (this could also be fixed by bumping MSRV to 1.92.0 and removing the unsafe blocks).

Fixes specialization syntax future-compatibility-warnings by moving the spec_traits module to a file (cc #427 ).

[alejandro-vaz]

for now I think we'll keep MSRV at 1.83, we might increase it in the future

you could implement the whole #427 modularization in this PR if you want

or open it on another

we can't fully merge modularization until we have #387 merged (I haven't reviewed it yet)

[alejandro-vaz]

I think the clear API boundary here is not spec_traits.rs, I'd honestly say it is:

• specialization.rs
• with all the methods that involve specialization
• lib.rs with a conditional compilation flag that includes specialization.rs only when the feature is enabled
• the normal methods on SmallVec on lib.rs that also have specialization have a conditional compilation flag of feature disabled

[alejandro-vaz]

this is gold