Fix error in Ops.mmap

[Kevin-Jin]

An error is thrown with the message environments cannot be coerced to other types when performing any comparison operation between mmap objects. The most likely culprit are the lines of the form coerceVector(compare_to,[...]) in mmap_compare() since compare_to is an mmap object and cannot be directly casted to vanilla vectors.

With all of its duplicated code, the mmap_compare() function also needs significant refactoring. I think that even using macros would make the code more readable and less error-prone to maintain.