Allow proper overriding of cfunctions in the core.

Allow overriding functions in the core libray to provide better
functionality on startup. Used to include our getline function in
the repl but use a simpler version in the core library.

src/boot/boot.c

@@ -43,7 +43,8 @@ int main() {
     /* Set up VM */
     int status;
     JanetTable *env;
-    env = janet_core_env();
+
+    env = janet_core_env(NULL);
 
     /* Run bootstrap script to generate core image */
     status = janet_dobytes(env, janet_gen_boot, janet_gen_boot_size, "boot.janet", NULL);