Simplify power by integer logic

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Refactor and reduce the number of multiplication.

Example calculation of x**16

x * x**8 * x**4 * x**2 * x # x**8, x**4, x**2 are calculate separately
# ↓
x * x * x**2 * x**4 * x**8 # result of x**4 is re-used to calculate x**8

Calculation of the largest x**n is dominant. Although multiplication is reduced, the performance improvement is limited.

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Before: 3**4 → (3 * (3**2)) * 3 → (3 * 9) * 3 → round(27) * 3 → 90
After: 3**4 → (3 * 3) * (3**2) → 9 * 9 → round(81) → 80

Although this test case seems to be improved, just a coincidence.
This pull request is not a bug fix. Large error still exist.

BigDecimal.limit 1
(BigDecimal('2')**100).to_i / 2**100 #=> 1577

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This is a small refactoring. It doesn't improve performance.
Closing this because the diff is not important and will be gone away in #347