Jam+murisi/feature/elixir resource machine

[murisi]

The target branch of this PR implements an interpreter for a Scheme subset. This PR implements a compiler for a variation of that DSL which is easier to compile to C. Specifically, the following has been implemented:

• A compiler from a Scheme representation using Elixir lists to C code
• Tail call optimization to guarantee that tail recursion never blows the stack (probably unnecessary because GCC -O3 has been able to detect and optimize all tail calls so far)
• Moved from using lists of string to lists of symbols to represent Scheme code. Doing this simplified the representation of string literals in the source Scheme programs
• Implemented variable renaming in order to avoid name clashes and avoid C reserved keywords from being used as variable names in the compiler's C output
• Anonymous functions (keyword: lambda) are replaced with named functions (keyword: function) to enable (mutually) recursive bindings. Furthermore these functions can be used as top-level definitions.
  • A key limitation of these anonymous functions is that they cannot be used after the function containing it returns because the function pointer essentially becomes a dangling pointer
• Used lexical scoping instead of dynamic scoping. This is easier to compile because GCC nested functions are also lexically scoped. More generally, lexical scoping is probably easier to compile to fast code because it allows stack frames and variable offsets to be fixed statically.

A way to use the compiler's C output inside RISC Zero is demonstrated at https://github.com/anoma/risc0-scheme . Essentially the C code is copied and pasted into https://github.com/anoma/risc0-scheme/blob/main/methods/guest/src/main.c , though it should be noted that the library functions provided in the compiled environment are slightly different to simplify the compiler outputs.