slicecompose v0.1.0

Initial release.

• compose(s1, s2): returns a slice equivalent to arr[s1][s2] for every sequence length, or None when no such slice exists — sound and complete in both directions.
• SliceChain: reduces chains of slices by adjacent-pair merging.
• SlicePlan / s_: lazy plan building with ordinary bracket notation, e.g. s_[2:][3:].slice == slice(5, None).

Pure Python, standard library only. See EXPLANATION.md for the soundness/completeness argument.