Review exposed internals

[Bodigrim]

It would be nice to review which parts of exposed internal modules proved to be useful and expose them through non-internal modules.

Also we need to discuss whether it is acceptable to carve out exposed internal from PVP guarantees (thus following suit of containers) starting from the next major release.

[andrewthad]

I'll throw two operators out there that do not currently exist in bytestring, but I need them sometimes, and it makes me reach for the internal modules:

-- | Makes a copy if the bytearray is unpinned. Aliases if it is pinned. Takes slicing metadata.
fromByteArray# :: ByteArray# -> Int -> Int -> ByteString
-- | Always aliases the byte array. Guaranteed to be O(1). Undefined behavior if bytearray is unpinned.
fromPinnedByteArray# :: ByteArray# -> Int -> Int -> ByteString

These are both important operators for people who want to build use MutableByteArray (from primitive), runST, and writeByteArray (probably writing out Word8s) to build ByteStrings. Currently, the public API makes this impossible.

[Bodigrim]

@andrewthad I think fromShort / toShort + slicing could fit the bill now.