Add the ext encode/decode

[akalend]

Please add the ext specification: encode/decode https://github.com/msgpack/msgpack/blob/master/spec.md#formats-ext The some code of encode/decode is https://github.com/akalend/hhvm-msgpack/blob/hhvm-v-3.12/msgpuck.h#L1515-L1540 and https://github.com/akalend/hhvm-msgpack/blob/hhvm-v-3.12/msgpuck.h#L1627-L1659 but it is not full realisation of specification: absent small objects 1,2,4 and 16 bytes.

So, must be the function mp_sizeof_ext(), mp_check_ext()

Thanks