Add support for new grammar in stack based Python decompiler

[frankhart2018]

Closes #8

Implementation

1. Push the constants in a constant pool.
2. If operations are performed then write them to string and push them to constant pool.
3. If print is requested then pop from this constant pool and print that with appropriate format specifier (according to left dtype, but this will change when there is implicit typecasting in future).
4. In (2) checked type for int or float and added the appropriate division symbol (// or /).