Add branching and more coherent method of function calls

We're turing-complete, babey!

* Call stack for functions now differentiates between native and quote
  calls
* Better API for builtin functions allowing for more control
* Example showing off branches

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile.rs

@@ -2,6 +2,7 @@ use crate::object::{QuoteTable, Value};
 use crate::scope::*;
 use crate::syn::ast::*;
 use crate::vm::inst::*;
+use std::rc::Rc;
 
 pub struct Compile<'s> {
     scope_stack: &'s mut ScopeStack,
@@ -35,7 +36,7 @@ impl<'s> Compile<'s> {
             // this gets compiled whenever it gets evaluated
             Expr::Quote(exprs) => {
                 self.scope_stack.push_scope();
-                let compiled = self.compile(exprs);
+                let compiled = Rc::new(self.compile(exprs));
                 let locals = self.scope_stack.pop_scope().unwrap();
                 let quote = self
                     .quote_table