Serialization protocol in C++

[jpivarski]

Since #68 is being scaled back to a Python function, C++ will need some serialization layer. awkward0.persist should be ported to C++.

Again, this is not for performance, but for accessibility.

[jpivarski]

Similar to the old one, but it doesn't have to be exactly the same. It really should be in C++, and that already limits the possibility of compatibility.

Also, Awkward1 has a different set of node types than Awkward0.

[jpivarski]

PR #348 added a serialization protocol in Python. There are two new functions, ak.to_arrayset and ak.from_arrayset, which turn an Awkward Array into a schema and a collection of flat arrays (an "arrayset"):

>>> original = ak.Array([[1, 2, 3], [], [4, 5]])
>>> form, container, num_partitions = ak.to_arrayset(original)
>>> form
{
    "class": "ListOffsetArray64",
    "offsets": "i64",
    "content": {
        "class": "NumpyArray",
        "itemsize": 8,
        "format": "l",
        "primitive": "int64",
        "form_key": "node1"
    },
    "form_key": "node0"
}
>>> container
{'node0-offsets': array([0, 3, 3, 5], dtype=int64),
 'node1': array([1, 2, 3, 4, 5])}
>>> print(num_partitions)
None

Unless there's a strong call for a C++ version, the serialization protocol in Python will be considered good enough.