Add feature of changing states

There are 6 states fow now including header, vardecl_list, vardecl,
body, il, st.
NOTE: can't find function get_identifier_token()
Just add some comment
Add function get_identifier_token()

NOTE:
 - In file lexer_utils.c, function get_identifier_toke() is a stub code
 - MUST change the source of lexer & parser from .c to .cc
 - NOT use file symtabl.* absyntax.* for now
Change the directory structure of project

Use CMake to generate Makefile automatically
Change the directory structure

Use CMake to generate Makefile automatically
Modify README.md