Coerce rational with the given prec in exp, log and power calculation

[tompng]

Follow up of #347
BigMath.exp(rational, prec) was converting Rational to BigDecimal with BigDecimal(rational, 0)
Change it to BigDecimal(rational, prec).

BigDecimal(8).power(1/3r, 40)
#=> 0.1999999999999999999999999999999986137056e1 (master)
#=> 0.2e1 (v3.2.2 and this branch)

Existing problem

Using prec to coerce is not sufficient in some case.

# Needs to coerce 30000001/3r with prec=1009 to calculate correct value
BigMath.exp(30000001/3r, 1000)