Remove read_expression and use parse_expression directly

lib/include/reader.h

@@ -5,7 +5,7 @@
 
 int read_line(int (*get_byte)(void), char *buffer, int buffer_size);
 
-const expression_t *
-read_expression(memory_pool_t *pool, const char *input, int len);
+int parse_expression(
+    memory_pool_t *pool, const char *input, int len, expression_t **out);
 
 #endif