can compile now....
add yyerror()
small change....
modify ast to fit contemporary 64-bit gcc
support idA := idB
add grammar file
fix mk_str
merge
Merge branch 'lab2' of github.com:alantian/pcat-compiler into lab2

Conflicts:
	src/pcat.y
add information on syntax
small change..
syntax-bnf->pcat.y
add abstruct syntax
syntax-bnf -> pcat.y
Merge branch 'lab2' of github.com:alantian/pcat-compiler into lab2
add unary-op & binary-op
modify abstruct syntax
Merge branch 'lab2' of github.com:alantian/pcat-compiler into lab2
small fix
Merge branch 'lab2' of github.com:alantian/pcat-compiler into lab2
modify abstrcut syntax