feat: Add feature flag allowing disabling rayon.

[MaxCWhitehead]

This change adds a feature flag enabling Rayon dep and the parallel iterator usage. This allows it to be disabled, for cases in which this is less performant.

Rayon feature is included in default features to prevent a change in behavior for current users of crate, though I expect perhaps having it off may be a more suitable default in the future.

Relates to #16