Pack CAR validity var, lower bound and upper bound in a single struct object

[feliperodri]

CBMC version: 5.67.0
Operating system: N/A

Function and loop contract instrumentation introduces three variables (car_valid, car_lb, car_ub) for each target specified in the assigns clause, for each local declaration, and for each assignment lhs. This greatly augments the number of objects created by CBMC during SymEx, and the number of object bits to use for pointers during the analysis.

Packing the three variables in a single struct would reduce the number of objects by 3 and save us one object bit.