Move structs used to hold custom operators and functions into new header

include/custom.h

@@ -0,0 +1,41 @@
+#ifndef LIBABACUS_CUSTOM_H
+#define LIBABACUS_CUSTOM_H
+
+/**
+ * A function pointer that is called
+ * to execute a certain type of function.
+ */
+typedef void(*libab_function_ptr)();
+
+/**
+ * A struct that holds informatiion
+ * about an operator that has been
+ * registered with libabacus.
+ */
+struct libab_operator_s {
+    /**
+     * The precedence of the operator.
+     */
+    int precedence;
+    /**
+     * The functionality of the operator.
+     */
+    libab_function_ptr function;
+};
+
+/**
+ * A struct that holds information
+ * about an function that has been
+ * registered with libabacus.
+ */
+struct libab_function_s {
+    /**
+     * The functionality of the function.
+     */
+    libab_function_ptr function;
+};
+
+typedef struct libab_operator_s libab_operator;
+typedef struct libab_function_s libab_function;
+
+#endif