Serialization of encryption context needs to check for single too-long key or value

[david-koenig]

Serialization format of encryption context does not allow any individual key or value to be longer than 2^16-1 bytes. Does serialization code check for this limitation? Do we have test vectors that detect this issue?

[mattsb42-aws]

Note for reference: this will implicitly throw an error when we try to serialize the value. We should instead throw a SerializationError with an appropriate message.

>>> import struct
>>> struct.pack('>H', 2**16 - 1)
b'\xff\xff'
>>> struct.pack('>H', 2**16)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: 'H' format requires 0 <= number <= 65535

[4gatepylon]

I've fixed this issue and made a PR at #188