Create a simple C++ embeddable Lisp

[oubiwann]

This will be a lot more fun than "just" using Chez Scheme (to be honest, that's actually a pretty daunting task ... with all the effort and time going into build setup and config; I'd rather spend that time doing fun coding, even if the end result takes longer).

This feature is part of the following epic:

• A debug REPL for signal processing #21

Template for development process:

• https://buildyourownlisp.com/contents
• https://github.com/orangeduck/BuildYourOwnLisp

Possible parsers:

• https://github.com/taocpp/PEGTL
• https://github.com/BitPuffin/sexpresso

Major tasks:

• Code layout
  • add a new subdir for the lisp in ./src
  • add a new subdir for the lisp in ./include
• REPL
  • Create REPL module for ft-lisp #24
  • extract the latest line from the widget
  • write a response to the next line followed by the prompt
• Parser
  • find a good parsing library and integrate into the build process
  • find or define a grammar for S-expressions or reverse polish notation
  • or use a library to both, in one go
• Memory protection
  • add max memory to use for stored environment
  • print warning message when max limit is close
  • print error message when max limit is reached
  • at MAX_HISTORY limit, do not allow for commands to be saved in history/display vecs
• Eval
• Error Handling
• S-expressions
• Garbage Collection
• Built-in Functions
• Variables
• Functions
• Conditionals
• Types

Once this is done, we can move onto the DSL:

• Define a VCV Rack DSL for an embedded Lisp #23

This Lisp flavour is not general purpose; it is specifically for embedding a DSL in a VCV Rack module. As such, there are features that aren't needed and won't be included:

• Quoting
• Macros

[oubiwann]

Possible s-expression parser:

• https://github.com/BitPuffin/sexpresso