[C++] Use a custom optional instead of `std::optional` for non-scalar type

[SirusDoma]

Is your feature request related to a problem? Please describe.
Currently bebop use concrete type when generating a field with non-scalar types. This make things simple since everything is value-based, but prevents user to make cyclic dependency which gracefully prevented in #111.

Describe the solution you'd like
I would like to suggest to revise and support cyclic dependency instead. Here what I had in mind:

1. Define a custom std::optional implementation, but instead of use value type, use heap (e.g smart pointer). This is relatively simple to do.
2. Determine definitions that are cyclic in parser, and then in the generator, forward declare them before generating the actual definitions.
3. The custom optional will incur extra cost in runtime; typically it use 4-8 bytes (the std::optional use extra 1 byte only) and it loss cache locality. Thus, this custom optional should only be used only for non-scalar type where the size is trivial

Describe alternatives you've considered
The easiest way (and relatively safe) to do this is to expose smart pointer for non-scalar type. But this would be a breaking change to the user.

Additional context
N/A

[SirusDoma]

For reference, Protobuf use pointer in the backing field, capnproto use it too (although I'm not sure how exactly it is implemented in capnproto). Both seems to allow user to have cyclic dependency