restore let-normalization, revise BIL normal forms

[ivg]

It looks like we have messed a bit with the normalization forms during the BAP 2.0 release. The let-normalization is not available anymore, despite being mentioned in the documentation and comments. So we need to restore let-normalizations and, probably, revisit the normal forms. I think we should split BNF2 into BNF2a and BNF2b (or BNF2 and BNF3) with one having the let-normalization and another not having.