Unbox a Haskell type and append the resulting bytes to a mutable byte
array. The array is grown exponentially when more space is needed.
Like
snoc except that the value is unboxed to the byte array.
Note: If you are serializing a large number of small fields, and the
types are statically known, then it may be more efficient to declare a
record of those fields and derive an
Unbox instance of the
entire record.