interpreter: BuiltIn overhaul!

- Allowed builtins to self-describe
- Broke builtins into their own module
  - Created a macro to work with BuiltIns easier
  - Uses macro 2.0 syntax, so it requires passing in ALL externally referenced identifiers
    - Luckily, this is already a requirement of good, readable macro definitions!
- As a temporary hack, turn overloadable operators into function calls
  - This is kind of pointless at the moment, since there can only be one definition of a function per name (no ADL/function overloading/traits/type namespaces yet!)
  - This is also pretty slow, but benchmarking shows it's not as slow as I thought (~400x slower in release mode than a native Rust implementation when running `fib.cl`/`fib.rs`. Totally unacceptable for most work, but this is a tree walk interpreter.)
  - TODO: Remove this when desugaring from operators to function calls is implemented

sample-code/fib.cl

@@ -1,12 +1,13 @@
 // Calculate Fibonacci numbers
 
-fn main() -> i128 {
-    let num = 10;
-    print("fib(", num, "): ", fib(num));
+fn main() {
+    for num in 0..=30 {
+        print("fib(", num, ") = ", fib(num))
+    }
 }
 
 /// Implements the classic recursive definition of fib()
-fn fib(a: i128) -> i128 {
+fn fib(a: i64) -> i64 {
     if a > 1 {
         fib(a - 1) + fib(a - 2)
     } else {