Make BigMath.exp and log also a module_method

[tompng]

BigMath methods are mostly module mehods, but exp and log were not (at least from v2.0.0).
There is no reason to exclude these two methods.

include BigMath
# These works
sin(3.14, 10)
cos(3.14, 10)
atan(1, 10)
# These were error
exp(1, 10)
log(2, 10)

I don't think include BigMath is a good idea, but it is used in bigdecimal's document and also in test codes.