Implement major math functions (#336)

* Move special value assertion methods to helper

* Implement BigMath.atan2(y, x, prec)

* Implement BigMath.hypot(x, y, prec)

* Implement BigMath.cbrt

* Implement asin and acos

* Implement hyperbolic functions(sinh, cosh, tanh)

* Implement inverse hyperbolic functions(asinh, acosh, atanh)

* Implement BigMath.log2 and BigMath.log10

* Implement BigMath.log1p and BigMath.expm1

* Add argument coerce assertion for all BigMath methods

Author: tompng