Support "tuples" and "lists" from SSZ Spec v0.5.1

[schroedingerscode]

@atoulme and/or @cleishm, if you wouldn't mind giving a little thought to the below proposal, I'd be most appreciative. I've spent a little bit mulling this over and plan to take a first stab at a PR shortly, but in the meantime an extra set of eyes doesn't hurt.

Description
The SSZ spec was rewritten as part of the eth2-specs v0.5.1 release to include both fixed-length and variable-length arrays/lists. Following the python notation used in the eth2-specs, and for the sake of clarity, fixed-length arrays/lists will be referred to as "tuples" and variable-length array/lists will be referred to as "lists".

(Not so) Quick Background

• Firstly, the SSZ spec has a limited number of base types:
  • uintN (where N is in 8, 16, 32, 64, 128, or 256)
    a N bit unsigned integer
  • bool
    A Boolean (True or False)
  • byte == uint8
    byte is an alias for uint8
• Composite types from these include:
  • tuple, represented as [type, N]
    a fixed-length homogenous collection of N values
  • list, represented as [type]
    a variable-length homogenous collection of values
• Some derived values, therefore are:
  • bytes == ["byte"]
    A variable-length collection of byte types (uint8 types by extension)
  • bytesN == ["byte", N]
    A fixed-length collection of byte types with N length

The implications of the above are that "list" requires a length mixin, where as a "tuple" does not.

Proposed Changes

• When Java bytes are serialized (i.e. when recursively serializing a Cava Bytes type), they should be serialized as a uint8.
• A Cava Bytes type of length 1 should be serialized as a uint8. [still undecided on this one]
• A Cava Bytes type of length >1 should be treated and serialized as a "list".
• All fixed length Cava BytesN types (i.e. Bytes32) should be treated and serialized as a "tuple".
• Support will be added for serializing Bytes... (varags list) as either a "list" or a "tuple"
  • The proposed method of achieving this is overloading the relevant functions with an additional length parameter.
• Support will be added for serializing List<? extends Bytes> as either a "list" or a "tuple".
  • The proposed method of achieving this is overloading the relevant functions with an additional length parameter.

Additional Information
See https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/simple-serialize.md.

[cleishm]

I must admit, I don't quite get this. UInt8 types are serialized as a single byte already. Large integers are serialized as little-endian byte arrays. Because SSZ contains no type information, you have to know what was serialized when you deserialize later.

A Bytes value has a variable length, so it is always serialized with a length prefix.

• When Java bytes are serialized (i.e. when recursively serializing a Cava Bytes type), they should be serialized as a uint8.

When serializing a Bytes type, each byte of the array will appear as a single byte in the serialized output - which is the same as a uint8.

• A Cava Bytes type of length 1 should be serialized as a uint8. [still undecided on this one]

I'd argue no on this, because it would make it impossible to use the SSZ API to create a serialization of a list of one byte.

• A Cava Bytes type of length >1 should be treated and serialized as a "list".

This is the current behavior.

• All fixed length Cava BytesN types (i.e. Bytes32) should be treated and serialized as a "tuple".

As above, I'd argue no on this.

• Support will be added for serializing Bytes... (varags list) as either a "list" or a "tuple"
• Support will be added for serializing List<? extends Bytes> as either a "list" or a "tuple".

The Cava SSZ API doesn't have methods for encoding/decoding sequences of known length (tuples), as these were added to the SSZ spec more recently. I agree these should be added. E.g. Bytes SSZ.encodeByteTyple(int length, Bytes value), Bytes SSZ.decodeByteTuple(Bytes source, int length).

[schroedingerscode]

Thanks for thinking through this one Chris. I made it down into the Cava SSZ weeds, and see most of what you're talking about. I'll concede that implicitly serializing things with/without the length mixin based on their type name might not be the best idea. I'll continue down the path of adding explicit API methods for fixed-length lists/tuples.