interpreter: Break out into a separate crate

My editor's performance was tanking because of macro interpreter::builtins::builtin!

Temporary solution: move the interpreter into a separate crate

If I intended to keep the interpreter around, in the long-term, it might be an idea to make a proc-macro for builtin expansion.
However, the only reason I need the macros is because the interpreter's dynamic typing implementation is so half-baked. After I bang out the new type checker/inference engine, I'll have to rewrite the entire interpreter anyway!

cl-interpret/examples/fib.rs

@@ -0,0 +1,16 @@
+// Calculate Fibonacci numbers
+
+fn main() {
+    for num in 0..=30 {
+        println!("fib({num}) = {}", fib(num))
+    }
+}
+
+/// Implements the classic recursive definition of fib()
+fn fib(a: i64) -> i64 {
+    if a > 1 {
+        fib(a - 1) + fib(a - 2)
+    } else {
+        1
+    }
+}