chore: remove const-generics feature impls

[austinabell]

MSRV is already past const generics (1.55.0), so these specialized impls can be removed. I haven't removed the feature itself to avoid being a breaking change.

I made this change in #104 but separated to here because it is an isolated change.

[matklad]

Let's maybe add MSRV to CI first? const-generics are stabilized piece-wise, so maaybe we are using something from 1.58 unadvertedly or something.

We should also update comment in Cargo.toml which explains what the cosnt-generics feature does (we have such a comment, right? ;) )

[matklad]

OTOH, this is an improvent, dosen' need to be blocked on anything.

[austinabell]

Let's maybe add MSRV to CI first? const-generics are stabilized piece-wise, so maaybe we are using something from 1.58 unadvertedly or something.

We aren't currently, msrv of this branch is 1.55.0 (same as on master without feature)

Linking to #106 (can add to CI in separate change)

We should also update comment in Cargo.toml which explains what the cosnt-generics feature does (we have such a comment, right? ;) )

I'm assuming this might just be removed on a major version bump (or maybe minor pre 1.0 is sufficient?). There isn't a reason otherwise to keep it