`Layer` definition with `Slice` compatibility

[kyleclo]

define Layer explicitly and make it such that doc.sentences: Layer has reasonable slicing behavior, so like doc.sentences[3:10] produces a Slice with a reasonable repl and ability to perform Entity-level operations on it in distributed maner, such as doc.sentences[3:10].text instead of [s.text for s in doc.sentences[3:10]]