toByteBuffer optimizations

[armanbilge]

1. toByteBuffer should only return a read-only buffer if necessary to protect its internals. Read-only buffers don't allow access to the underlying array which can force further copies downstream. If the data is already being copied, then there is no reason to return a read-only buffer.

2. Add toByteBufferUnsafe. In the spirit of toArrayUnsafe, but can avoid copies in several more situations e.g. a view of a slice of an array. This will enable copy-free transforms to Chunk.ArraySlice in FS2.