Improve serialization

[BitPuffin]

Users of the library have ended up replacing the serialization in sexpresso because it doesn't
necessary format the output the way that they want to. Perhaps there is a way that I can give them
some control over how the output is formatted.

Furthermore perhaps it would be useful to provide a streamlined way to serialize data structures. My current thinking is to provide a tool that reads an s-expression spec file for a given data structure and spits out C++ code that one compiles with the project. So when buildding the project, running the spec compiler on those files to generate the C++ code would be a dependency you do first run before compiling the project c++ code. Therefore it is crucial that this compiler is fast, and that the code it generates can compile quickly. However a problem with this approach is that if the user updates their data structure then the spec needs to be updated as well, so this creates a maintenance burden. I wonder if there is any way I can make it automatic and easy to maintain but with options for further granular control.