Improvements to log1m_exp:
Fixes wrong promotion in fwd-diff.
Fixes formatting in the function.
Some improvements to log_diff_exp:
Fixes wrong promotion in fwd-diff;
Improves formatting in rev-diff;
Fixes wrong doc in math;
Fixes math unit test.
Improvement to ordered_logistic distribution: now it uses log1m_exp(.) instead of log1m(exp(.)).