String Instructions

The following instructions are modified to act on strings:
- INDEX: Now pulls a char out at index
- SUBSL: Now pulls a substring out of a source string
- INSER: Now inserts a char into a string at index
- LENGTH: Now returns length of a string

In addition to the above instructions, additional instructions
are now implemented to handle strings:
- CONCAT: appends a string onto another string
- S_APPEND: appends a char on to the end of a string

Fixes: #38

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

hyphae/src/heap.rs

@@ -17,7 +17,6 @@
 
 use core::ops::{Index, Deref, DerefMut};
 use core::ptr::NonNull;
-use core::cell::RefCell;
 
 use alloc::rc::Rc;
 use alloc::vec::Vec;
@@ -141,8 +140,8 @@ pub enum Datum {
     Symbol(String),
     Char(u8),
     String(Vec<u8>),
-    Vector(RefCell<Vec<Gc<Datum>>>),
-    ByteVector(RefCell<Vec<u8>>),
+    Vector(Vec<Gc<Datum>>),
+    ByteVector(Vec<u8>),
     None
 }