Add experimental KV type wrapper

[eddyashton]

Opening as a draft while I clean up some TODOs.

Continues refactor of KV, adding kv::experimental:: alternatives to kv:: for Map and TxView types. The experimental:: variants wrap untyped (ie - std::vector<uint8_t> -> std::vector<uint8_t>) instances of the old types, exposing the same interface (not just implementing the same Abstract types, but also matching template-discovered public typedefs) but implementing serialisation inside get/put etc.

The eventual goal is to use these maps as standard, so kv::Map will become kv::untyped::Map, and kv::experimental::Map will become kv::Map, with no visible API changes. For now they are experimental while I test the compatibility, and next I'll do a rollout testing them in existing Maps.

[ccf-bot]

experimental_kv_type_wrapper@8714 aka 20200527.5 vs master ewma over 50 builds from 8543 to 8711

[jumaffre]

LGTM!