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/src/eval.rs

@@ -15,6 +15,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+use alloc::boxed::Box;
 use crate::segment::{Ctr, Seg};
 use crate::sym::{SymTable, call_lambda};
 use crate::error::Traceback;