Introduce status_flags::{is_positive,is_negative,purely_indefinite}

Introduce status_flags::{is_positive,is_negative,purely_indefinite}

Quite a number of transformations can be applied if the expression
is known to be positive (negative) (for instance, sqrt(x^2*y^2) = x*y
if x >= 0 and y >= 0). However the check itself might be quite expensive.
Hence a few status_flags are introduced to cache the result.