BigMath.exp is slow for large argument.

[mrkn]

The original issue is https://bugs.ruby-lang.org/issues/3270

It can be optimized for the large argument by the following formula.

exp(a * b + c) = exp(a)**b * exp(c)

[tompng]

Fixed by #347

BigMath.exp(100000, 100).to_i
# 0.402142s → 0.001299s
BigMath.exp(1e19, 100)
# Infinite seconds → 0.000842s