Read interface for new serialization system

[vtnerd]

This is the read interface for my proposed serialization system, along with the remainder of the (major) adapters/wrappers that were not in the write interface review. After this review are the implementations (epee, json, msgpack), and then the changes needed to the existing code.

[jeffro256]

What will basic_value be used for?

[vtnerd]

basic_value is used by JSON-RPC id field, and no one where else currently.

[vtnerd]

@jeffro256 Earlier I mentioned protobuf, and I forgot completely that this interface cannot support reading of protobuf arrays (without some modifications to the interface). Write support should be possible (although I haven't tried to implement it yet). I've otherwise implemented reading/writing of everything else protobuf.

I think its better to introduce the necessary changes when/if protobuf is added as potential format. A couple of people have asked for it years ago, but its pointless unless we can auto-generate schemas from this interface (another thing I failed to figure out - at least yet).

Otherwise, epee_binary, json, msgpack are all implemented completely, and CBOR should be possible as well.

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Reading Protobuf Arrays

Reading protobuf arrays is difficult because a field within an object appears multiple times, and does not have to be contiguous. The binary format looks something like:

  {"1": 100, "2": "string", "1": 300}

which specifies an integer array of length 2: [100, 300].

[vtnerd]

@jeffro256 I finally updated this PR. I thought it was done previously, but I didn't merge in the requested changes for the read interface.

Changes:

• Rebased
• Changes requested by @jeffro256
• Add API support for reading integer/msgpack keys