Make parsing function public.

libab already provides a _run_tree ... you can't run_tree if there's
no tree.

include/libabacus.h

@@ -232,6 +232,13 @@ void libab_get_false_value(libab* ab, libab_ref* into);
  */
 void libab_get_bool_value(libab* ab, int val, libab_ref* into);
 
+/**
+ * Parses the given piece of code using the given libabacus instance.
+ * @param ab the instance to use to parse the code.
+ * @param string the source code to parse.
+ * @param into the value to store the newly parsed tree into.
+ */
+libab_result libab_parse(libab* ab, const char* string, libab_tree** into);
 /**
  * Executes the given string of code.
  * @param ab the libabacus instance to use for executing code.