mallocing slows down cex mode

[priyasiddharth]

Regarding the common helper functions... For a specific example: mallocing nodes in the helper functions seems to increase cex mode run time from 1s to 14s. Stack allocating in the harness itself and using helper functions to initialize puts the cex mode runtime at 3s. My recommendation is to not malloc in the helper function. If sounds ok... then i will refactor.

info:
target commit: 54dad4d

job: linked_list_push_back

original run time in cex mode: 1s
new run time in cex mode: 14s
new run time if malloc removed by stack allocation in harness: 3s

vac and normal mode are not affeced