C++ struct writer

[tiffany1618]

Closes #137 Closes #138

Added

• StructCppWriter: Writes C++ for FPP struct definitions
• Unit tests for C++ struct writer

Changes from Python XML struct autocoder:

• Add const and non-const getters for arrays and serializable type members, excluding enum types, which are passed by value (resolves issue of getting and then setting a struct member; e.g. a pattern like s.get_a().set_b(), where s is a struct containing a struct type member a, which in turn contains another member b)
• Pass array type members by C++ reference instead of by pointer and size (simpler, enforces checking of array length)
• Remove pointer copy constructor (reference copy constructor is sufficient)

[tiffany1618]

fpp-to-xml gives an error for empty structs, but fpp-to-cpp currently doesn't. Do we want fpp-to-cpp to also give an error?

[bocchino]

fpp-to-xml gives an error for empty structs, but fpp-to-cpp currently doesn't. Do we want fpp-to-cpp to also give an error?

No. Empty structs are allowed in FPP and in C++. The error in fpp-to-xml exists because the XML schema does not support them. So this is a case where we are implementing part of the FPP spec that is not implemented when going through XML.

[bocchino]

Also, same comment as on #144 -- please re-run all unit tests and update them as necessary.