Pave the way for backend plugins

[andrew-johnson-4]

create a stable environment for compiler backends to move forward.

For compiler backends:

• "only" use stable internal compiler APIs (there are NOT very many currently)
  • https://github.com/andrew-johnson-4/lambda-mountain/wiki/Internal-Compiler-API
• "only" use C and LSTS syntax that is documented here
  • https://andrew-johnson-4.github.io/lsts-language-reference/
  • plugin needs to be registered with a backend hook
• backend hooks don't exist yet (would not be too hard to do, just haven't gotten to it)
  • Hooks to backend by configuration #1113