Calculate division and multiplication with Ruby's Integer for large precision

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Ruby's Integer implements faster algorithm (karatsuba, toom3, GMP)
Using Integer's div/mult needs base conversion, but it's still faster when precision is large.

# Mult threshold
prec=150; x=BigDecimal('7'*9*prec); 10000.times{x*x}
# processing time: 0.867585s
prec=149; x=BigDecimal('7'*9*prec); 10000.times{x*x}
# processing time: 1.041640s

# Div threashold
prec=299; y=BigDecimal('7'*9*prec); 10000.times { BigDecimal(1).div(y, 9*prec) }
# processing time: 1.426509s
prec=300; y=BigDecimal('7'*9*prec); 10000.times { BigDecimal(1).div(y, 9*prec) }
# processing time: 1.299771s

# Performance degrade case.
x = BigDecimal('1'*9*300); (x*7).div(x, 9*290) #=> 0.7e1
# processing time: 0.016612ms
x = BigDecimal('1'*9*300); (x*7).div(x, 9*300) #=> 0.7e1
# processing time: 0.209515ms