Comparison property builder combinator

[fortunac]

Currently for our comparison analysis, different postconditions are built separately in with compare_subs_eq and compare_subs_fun in https://github.com/draperlaboratory/cbat_tools/blob/master/wp/lib/bap_wp/src/compare.ml.

We should add combinator in order to build postconditions that compare multiple properties (ex: compares both the register values and the function calls), giving the user the ability to pick multiple.

We should also move all the variable computation from

cbat_tools/wp/plugin/wp.ml

Line 111 in 9f1180b

let compare_projs (proj : project) (file1: string) (file2 : string)

into the compare module.

[fortunac]

Closed in #158.