Implement constant folding

[erezsh]

This doesn't fold arithmetic expressions, because there are nuances there that need to be decided. Like, is there overflow, and does division of ints return int or float?

Solves issue #32

[aoskotsky-amplify]

Hey thanks for the PR! I'm not sure if this should be the default behavior. There might be some cases where users want to work with HCL2 exactly as is. What do you think about having an optional flag argument on the load methods to enable/disable folding? It might require creating 2 different transformers and flipping between them based on the flag.

[htorianik]

Hi @erezsh! First thing first, thank you very much for contributing. The solution you provided is not a complete constant folding, as it doesn't test for non-predicate arithmetic operations. We believe, that the convenient solution is to:

1. Modify the grammar so arithmetical and logical operators will be with the right priorities, so the lexer can build a valid syntax tree.
2. Create 2 different transformer classes, one of them will test expressions for constant folding.
   It's a time-consuming task, which we put into our backlog, but for now, we are going to close it.

And again, thanks for contributing to the project.