mark core as nostd

* implement custom hashmap to back symtable
* pass in print and read callbacks to keep stdlib pure
* use core / alloc versions of Box, Rc, Vec, etc
* replace pow func with libm

Signed-off-by: Ava Affine <ava@sunnypup.io>

core/tests/test_eval.rs

@@ -102,7 +102,7 @@ mod eval_tests {
         let rh_doc = "(apply (lambda (x) (car x)) (1 2 3))";
 
         let mut syms = SymTable::new();
-        static_stdlib(&mut syms);
+        static_stdlib(&mut syms, |_: &String| (), || String::new());
 
         eval(&lex(&comparator.to_string()).unwrap(), &mut syms).unwrap();
         assert_eq!(