ByteString conversions

[jac3km4]

Added the ByteString conversions I've slightly changed bytesFromByteString, I think it should be better now Also, I've added it to Std.Foreign.PrimArray, because putting it in Std.Vector.Base would cause circular module dependencies, since Std.Foreign.PrimArray uses the vector modules.

[winterland1989]

It's OK to add these to Std.Foreign.PrimArray, but I still want to avoid copying in those functions if possible. You can do it by directly match on PrimVector or ByteString's contructor, get the ByteArray# (or MutableByteArray# if possible):

• If it's ByteArray# coming from Bytes, depend on it's pinned or not we can save a copying.
• If it's MutableByteArray# from ByteString, we can directly freeze it and construct a Bytes, if it's another type of ForeignPtr payload, we do a copy.

I know it sounds complicated, but It's probably better than doing copy and allocation everytime, since:

• Now days most of ByteStrings are based on MutableByteArray#.
• Most of time, Bytes coming from IO operations are pinned.