Support serialization of fixed-size arrays in abi_serializer.

[greg7mdp]

[partially] resolves #1931.

• add support for serialization of fixed size array types such as uint8[5].
• [perf] add missing reserve() when deserializing a vector (or now an array), fix unnecessary copy in vector<fc::variant> vars = var.get_array();
• add tests (thanks to Kevin) for serialization of fixed size array types.

[greg7mdp]

I think we should check that the length of the array passed to serialize matches the size in the rtype.

[spoonincode]

I'm missing some context to this change so I may be misunderstanding what the goal is here. Fixed sized arrays have (annoyingly imo) been an omission of the ABI spec. My first thought is if cdt is generating a uint8_t[96] type in an ABI, it's generating a defective ABI. Such an ABI would be disallowed in, for example, abieos,
https://github.com/AntelopeIO/abieos/blob/main/src/abi.cpp#L58
and we'd have to investigate eosjs, wharf, etc too. Are we trying to add support in the ABI spec for type[size]?

[greg7mdp]

Are we trying to add support in the ABI spec for type[size]?

I'll let Kevin answer this one. Regardless of whether fixed-size arrays are officially supported in the abi, I don't think it would hurt to have these supported in abi_serializer.