Input BNF

[KnowingNothing]

Here are known issues, we should fix these issues for next year project:

1. Reduce should be to- level, just as TVM
2. No inplace operation, it's too ambiguous (for example, A[i, j] = A[j, i]), or mark it in json explicitly
3. for expressions, introduce priority to operators (+,-,*,/)
4. allow constant indices