Model integers as bounded

[alegnani]

Pancake uses bounded integers of 64-bits (32-bit for 32-bit binaries). This is currently not modeled in Viper where they are instead unbounded.

• [ ] Add support for unsigned logical operators: <+, <=+, >+, >=+
• [ ] Add checks and constraints to integers in Viper

[alegnani]

Fixed in #52