add __abs__ for cfix and sfix

[lance6716]

tested with these code

a = cfix(0.02)
b = cfix(-0.02)
c = sfix(0.02)
d = sfix(-0.02)

test(abs(a), a)
test(abs(b), -b)

test(abs(c), a)
test(abs(d), -b)

[mkskeller]

This is not very efficient. Keep in mind that comparisons are relatively expensive, and it's not necessary to do two comparison just to find out the sign of a value.

[lance6716]

Could there be a data type of secret bool for comparison? Such as GF(p) and GF(2^n) of n = 1 and p is huge.