Run the builder to construct a strict bytestring containing the
sequence of bytes denoted by the builder. This is done by first
serializing to a lazy bytestring and then packing its chunks to a
appropriately sized strict bytestring.
toByteString = packChunks . toLazyByteString
Note that
toByteString is a
Monoid
homomorphism.
toByteString mempty == mempty
toByteString (x `mappend` y) == toByteString x `mappend` toByteString y
However, in the second equation, the left-hand-side is generally
faster to execute.