Improvements to debug output.

Renamed monitor_debug_parse_tree() to monitor_debug_print_parse_tree().
Renamed monitor_debug_parse_node() to monitor_debug_found_parse_node().
Added similar function monitor_debug_added_to_parse_node() called when
appending a child to a list-type terminal.

Removed the prefix parameter from lexer_scan_next_buffer() and
monitor_debug_parse_node().

Increased the indent for lemon parser trace prefixes.