Investigate parsing sub-members as member

[beached]

Investigate allowing the parsing of submembers. e.g

{ "a": { "b": 5 } }

The transformation would be potentially lossy

Possible mapping syntax

json_submember<"a", "b", json_number_no_name<int>>

Where "b" can use the path syntax, e.g "b.c" would mean a class like

{ "a": { "b": { "c": 5 } } }