Specialize Chunk#toArraySlice

[armanbilge]

1. Specializes Chunk#toArraySlice for ByteVector and {Byte,Char}Buffer to avoid array copies.
2. Fix class cast exceptions in Chunk.{Byte,Char}Buffer#toArray.
3. Avoid converting buffers to read-only, since this prevents access to underlying array.
4. Use toArraySlice to implement to{Byte,Char}Buffer

[armanbilge]

• primitive classtag equality broken in 2.12 scala-native/scala-native#3244

[armanbilge]

Actually, maybe we shouldn't be comparing ClassTags for equality at all.

[armanbilge]

@diesalbla I'd appreciate it if you didn't squash-merge my PRs 😕 I worked carefully to split this PR into a series of easy-to-review commits.