PERFORMANCE-NOTES

Just some notes on performance with Python 3.5. We are taking these into
account in the code.

- 60% faster to create lists with [] list comprehensions than tuples
  or lists with tuple(), list().  Of those list is 10% faster than
  tuple.

- however when not initializing from a generator, a fixed-length tuple
  is at least 80% faster than a list.

- an implicit default argument is ~5% faster than passing the default
  explicitly

- using a local variable x rather than self.x in loops and list
  comprehensions is over 50% faster

- struct.pack, struct.unpack are over 60% faster than int.to_bytes and
  int.from_bytes.  They are faster little endian (presumably because
  it matches the host) than big endian regardless of length. Furthermore,
  using stored packing and unpacking methods from Struct classes is faster
  than using the flexible-format struct.[un]pack equivalents.

  After storing the Struct('<Q').unpack_from function as unpack_uint64_from,
   later calls to unpack_uint64_from(b, 0) are about 30% faster than calls to
   unpack_from('<Q', b, 0).


- single-item list and tuple unpacking.  Suppose b = (1, )

   a, = b  is a about 0.4% faster than  (a,) = b
           and about 45% faster than a = b[0]

- multiple assignment is faster using tuples only for 3 or more items

- retrieving a previously stored length of a bytes object can be over 200%
  faster than a new call to len(b)