Function flatten 

pub(crate) fn flatten<S: Simd>(
    simd: S,
    path: impl IntoIterator<Item = PathEl>,
    tolerance: f64,
    callback: &mut impl Callback,
    flatten_ctx: &mut FlattenCtx,
)

See the docs for the kurbo implementation of flattening: https://docs.rs/kurbo/latest/kurbo/fn.flatten.html

This version works using a similar approach but using f32x4/f32x8 SIMD instead.