Add grammar.cpp grammar.h.
Add hash.cpp hash.h.
Add set.cpp set.h
Move function nullable to grammar.cpp. Add function first(), follow().
Add symbol_table in struct Grammar.
Fix a typo.
Modify parser_gen to let it accept struct Grammar as argument.
Symbol epsilon will be nullable now.
Fix how fumction follow() works.
Add Makefile and lltable related source code.
output to set.txt instead of stdout.
Add main.cpp.
Remove debug code.
Merge branch 'parser_gen'