Chunk support for ArraySeq

[tmccarthy]

This PR adds support for immutable and mutable ArraySeqs for Chunk. Conversions in both directions are provided. This PR follows on from #1899.

Constructing a Chunk from ArraySeq

The Chunk.arraySeq factory method constructs a Chunk by wrapping the underlying array. This reproduces the behaviour previously implemented in ArrayBackedSeq.

import scala.collection

Chunk.arraySeq(mutable.ArraySeq(1, 2, 3))   // Create Chunk from mutable ArraySeq
Chunk.arraySeq(immutable.ArraySeq(1, 2, 3)) // Create Chunk from immutable ArraySeq
Chunk.iterable(immutable.ArraySeq(1, 2, 3)) // Delegates to the factory methods above

Constructing an ArraySeq from a Chunk

import scala.collection.immutable.ArraySeq

val chunk = Chunk(1, 2, 3)

chunk.toArraySeq         // Uses implicit ClassTag to specialise ArraySeq
chunk.toArraySeqUntagged // Creates untagged ArraySeq

Why provide specialised support for ArraySeq?

The immutable ArraySeq is newly added in Scala 2.13. It is an immutable wrapper over a bare array with specialised implementations for primitives. As such it is well suited to the kinds of performance-sensitive uses for which fs2 is commonly used.

[tmccarthy]

I have replaced the previous implementation of this behaviour via ArrayBackedSeq (now deleted).

[tmccarthy]

This seems to be the most efficient way to construct an ArraySeq, given it will often take advantage of System.arraycopy. I might be missing something, though, so I'm open to feedback if there are better ways to do this.

[tmccarthy]

We attempt to specialise the ArraySeq even if there is no ClassTag available.

[tmccarthy]

This comment replaces the now-deleted comment here in Chunk.

[tmccarthy]

Removing the ArrayBackedSeq matcher has broken the build for 2.12. I'll need to re-think the cross-version approach for the .iterable method 😕.

[mpilquist]

Can you make this trait and it’s companion private[fs2] both here and in 2.13 file? Otherwise looks ready for merge. Thanks!

[tmccarthy]

Thanks @mpilquist! This one should be good to go once the build completes.